Goliath was offspring of a tainted bloodline. Tainted by Nephilim (Fallen Angels)
You can find more info on this by reading the Book of Enoch, which is still accepted in certain Christian denominations, such as in Ethiopia.
Although evidently widely known during the Development of the Hebrew Bible canon, 1 Enoch was excluded from both the formal canon of the Tanakh and the typical canon of the Septuagint and therefore, also the writings known today as the Deuterocanon. One possible reason for Jewish rejection of the book might be the textual nature of several early sections of the book that make use of material from the Torah; for example, 1 En 1 is a midrash of Deuteronomy 33. The content, particularly detailed descriptions of fallen angels, would also be a reason for rejection from the Hebrew canon at this period. Another reason it was removed may be because of it's harsh descriptions against the Jews and Romans. Additionally, it talks about Jesus, a Messiah called "Son of Man", with divine attributes, generated before the creation, who will act directly in the final judgment and sit on a throne of glory (1 Enoch 46:1–4, 48:2–7, 69:26–29)
1 Enoch Chapter 7 reads:
And all the others together with them took unto themselves wives, and each chose for himself one, and they began to go in unto them and to defile themselves with them, and they taught them charms and enchantments, and the cutting of roots, and made them acquainted with plants. And they became pregnant, and they bare great giants, whose height was three thousand ells: Who consumed all the acquisitions of men. And when men could no longer sustain them, the giants turned against them and devoured mankind. And they began to sin against birds, and beasts, and reptiles, and fish, and to devour one another's flesh, and drink the blood. Then the earth laid accusation against the lawless ones.
Enoch Chapter 10 reads:
2 '〈Go to Noah〉 and tell him in my name "Hide thyself!" and reveal to him the end that is approaching: that the whole earth will be destroyed, and a deluge is about to come upon the whole earth, and will destroy all that is on it. 3. And now instruct him that he may escape and his seed may be preserved for all the generations of the world.'
8 And the whole earth has been corrupted through the works that were taught by Azâzêl: to him ascribe all sin.' 9. And to Gabriel said the Lord: 'Proceed against the bastards and the reprobates, and against the children of fornication: and destroy [the children of fornication and] the children of the Watchers from amongst men [and cause them to go forth]: send them one against the other that they may destroy each other in battle: for length of days shall they not have.
So we see here how these giants in the pre-flood were dealt with, however, post-flood there were also giants.
A Second Irruption of Giants
Where did these giants come from? Since all the Pre-Flood giants were destroyed by each other, or before the Flood, these must have been born to disobedient angels who sinned by cohabiting with women soon after the Deluge. Says the Companion Bible:
"But we read of the Nephilim again in Num.13:33: 'there we saw the Nephilim, the sons of Anak, which come of the Nephilim.' How, it may be asked, could this be, if they were all destroyed in the Flood? The answer is contained in Gen.6:4, where we read: 'There were Nephilim in the earth in those days (i.e., in the days of Noah); and also AFTER THAT, when the sons of God came in unto the daughters of men, and they bare children unto them, the same became the mighty men (Heb. gibbor, the heroes) which were of old, men of renown' (lit. men of the name, i.e. who got a name and were renowned for their ungodliness).
"So that 'after that,' i.e. after the Flood, there was a second irruption of these fallen angels, evidently smaller in number and more limited in area, for they were for the most part confined in Canaan, and were in fact known as the 'nations of Canaan.' It was for the destruction of these, that the sword of Israel was necessary, as the Flood had been before.
"As to the date of this second irruption, it was evidently soon after it became known that the seed was to come through Abraham; for, when he came out from Haran (Gen. 12:6) and entered Canaan, the significant fact is stated: 'The Canaanite was then (i.e. already) in the land.' And in Gen.14:5 they were already known as 'Rephaim' and 'Emim,' and had established themselves at Ashteroth Karnaim and Shaveh Kiria- thaim" (Appendix 25).
Satan knew that the promised Messiah would come through the descendants of Abraham. Satan adopted a pre-emptive strategy and bred more "giants" to occupy the Promised Land to prevent the Israelites from being able to occupy it. His plan aborted, however, when the Israelites under Moses and later Joshua conquered the inhabitants of the land, and destroyed the remaining Anakim.
Another Race of Giants -- the Rephaim
In the book of Deuteronomy, Moses writes of the land of Moab, "The Emims dwelt therein in times past, a people great, and many, and tall, as the Anakims; which also were accounted giants, as the Anakims" (Deut.2:10-11). The word for "giant" here is Raphah, which means "giant." Interestingly, the same word also means "ghost, dead, deceased." The Rephaim were another of the lines of giants which existed after the Flood. Moses tells us of the land of Moab, "That also was accounted a land of giants; GIANTS dwelt therein in old time; and the Ammonites call them Zamzummims; a people great, and many, and tall, as the Anakims; but the LORD destroyed them before them; and they succeeded them, and dwelt in their stead" (v.20-21).
Og, the king of Bashan, conquered by Moses and the Israelites on their entrance into the Promised Land, was one of the giants. "For only Og king of Bashan remained of the remnant of giants; behold, his bedstead was a bedstead of iron; is it not in Rabbath of the children of Ammon? nine cubits [approximately 14 feet!] was the length thereof, and four cubits [6 feet] the breadth of it, after the cubit of a man [i.e., a normal man -- the normal cubit measured 18 inches]" (Deut.3:11). Bashan, the land of Og, was called "the land of giants"(v.13; see also Josh.12:4, 13:12). The valley of Hinnom, the refuse dump for Jerusalem, a type of the final Gehenna fire, lay eastward of the "valley of the giants" (Josh.15:8; 18:16). Joshua 17:15 mentions the "Rephaim," another branch of the "giants." These dwelt in the region near Sodom and Gomorrah in the time of Abraham (Gen.14:5; 15:20; see also II Sam.5:18,22; 23:13).
In an ancient text of the Jews, we read an astonishing description of some of these gigantic Amorites whom the Israelites conquered. In Buber's Tanhuma, Devarim 7, the text tells us of a Rabbi Johnanan ben Zakkai's encounter with the Roman Emperor Hadrian. This event occurred in about A.D. 135, soon after the Roman victory in the Bar Kochba war, when the Jews rebelled against the Romans. The text reads:
"The wicked emperor Hadrian, who conquered Jerusalem, boasted, 'I have conquered Jerusalem with great power.' Rabbi Johanan ben Zakkai said to him, 'Do not boast. Had it not been the will of Heaven, you would not have conquered it.' Rabbi Johanan then took Hadrian into a cave and showed him the bodies of Amorites who were buried there. One of them measured eighteen cubits [approximately 30 feet] in height. He said, 'When we were deserving, such men were defeated by us, but now, because of our sins, you have defeated us'" (quoted in Judaism, edited by Arthur Hertzberg, p.155-156, George Braziller, New York: 1962).
Thirty feet tall! The bones of these men were still in existence during the time of Josephus, the Jewish historian of the first century!