According to the Biblical record, Moses was alone with God, with no man with him, when he died--as commanded by God.
48And the LORD spake unto Moses that selfsame day, saying,
49Get thee up into this mountain Abarim, unto mount Nebo,
which is in the land of Moab, that is over against Jericho; and behold
the land of Canaan, which I give unto the children of Israel for a
possession: 50And die in the mount whither thou goest up,
and be gathered unto thy people; as Aaron thy brother died in mount
Hor, and was gathered unto his people: 51Because ye
trespassed against me among the children of Israel at the waters of
Meribah-Kadesh, in the wilderness of Zin; because ye sanctified me not
in the midst of the children of Israel. 52Yet thou shalt
see the land before thee; but thou shalt not go thither unto the land
which I give the children of Israel. (Deuteronomy 32:48-52, KJV)
After being commanded to go to die, the record states the following:
1And Moses went up from the plains of Moab unto the
mountain of Nebo, to the top of Pisgah, that is over against Jericho.
And the LORD showed him all the land of Gilead, unto Dan, . . .
4And the LORD said unto him, This is the land which I sware
unto Abraham, unto Isaac, and unto Jacob, saying, I will give it unto
thy seed: I have caused thee to see it with thine eyes, but thou shalt
not go over thither. 5So Moses the servant of the LORD
died there in the land of Moab, according to the word of the LORD.
6And he buried him in a valley in the land of Moab, over
against Bethpeor: but no man knoweth of his sepulchre unto this day.
7And Moses was an hundred and twenty years old when he
died: his eye was not dim, nor his natural force abated.
8And the children of Israel wept for Moses in the plains of
Moab thirty days: so the days of weeping and mourning for Moses were
ended. 9And Joshua the son of Nun was full of the spirit
of wisdom; for Moses had laid his hands upon him: and the children of
Israel hearkened unto him, and did as the LORD commanded Moses.
10And there arose not a prophet since in Israel like unto
Moses, whom the LORD knew face to face, 11In all the signs
and the wonders, which the LORD sent him to do in the land of Egypt to
Pharaoh, and to all his servants, and to all his land,
12And in all that mighty hand, and in all the great terror
which Moses showed in the sight of all Israel. (Deuteronomy 34:1-12,
Now there are some obvious problems with that final chapter, from a human perspective. Seeing as no one but God is recorded as present with him, no one knows where Moses was buried, and it was God who buried him, we have no human witness of the event. Further, if Moses did write this chapter, how could he have known what would take place after his death? But if Moses was not the writer, how could the writer have known where Moses went and what had become of him? Either way one approaches this, the only possibility for having a reliable source for the information is to have received it from God, who both sees the future and knows all.
In either case, there is no record for the manner of Moses' death beyond that God was with him at that time and it was at God's behest that he had gone there into the secluded mountain to die. And God Himself buried Moses.
In an interesting twist, perhaps after three days in his tomb to symbolize the resurrection of Christ, for whom Moses had been a type, God then sends Michael to raise Moses to life, as recorded in the book of Jude.
Yet Michael the archangel, when contending with the devil he disputed
about the body of Moses, durst not bring against him a railing
accusation, but said, The Lord rebuke thee. (Jude 1:9, KJV)
Moses is seen in the New Testament as speaking with Jesus on the mount of transfiguration.
And there appeared unto them Elias with Moses: and they were talking
with Jesus. (Mark 9:4; cf. Matthew 17:3; Luke 9:30)
So sometime shortly after Moses had died, he was resurrected and taken to Heaven.
Nothing in the record indicates the cause of Moses' death, beyond the fact that God had ordered it, and since no human was there present to witness it there is no possible way of knowing its cause. Any human tradition or belief about it amounts to pure speculation. We simply do not, and cannot, know.