It is helpful to read all of Exodus 4 starting at verse 18.
After the burning bush, and God giving Moses signs to do in Egypt, Moses then asked his father in-law if he could return to Egypt. Moses took his wife and two sons, put them on a donkey, and set off for Egypt.
Notice verse 24: ON THE WAY.
Moses' wife was not an Israelite and she was NOT going to allow her son to be circumcised. Moses had yielded to the wishes of his wife, but as he was on the way to represent the God of Abraham, Isaac and Jacob, God shows us that NOBODY was exempt from His covenant:
9 And God said to Abraham: “As for you, you shall keep My covenant,
you and your descendants after you throughout their generations. 10
This is My covenant which you shall keep, between Me and you and your
descendants after you: Every male child among you shall be
circumcised; 11 and you shall be circumcised in the flesh of your
foreskins, and it shall be a sign of the covenant between Me and you.
12 He who is eight days old among you shall be circumcised, every male
child in your generations, he who is born in your house or bought with
money from any foreigner who is not your descendant. 13 He who is born
in your house and he who is bought with your money must be
circumcised, and My covenant shall be in your flesh for an everlasting
covenant. 14 And the uncircumcised male child, who is not circumcised
in the flesh of his foreskin, that person shall be cut off from his
people; he has broken My covenant.” (Genesis 17:9-14)
When Zipporah saw that God was going to kill Moses, she defiantly complied.
24 And it came to pass on the way, at the encampment, that the Lord
met him and sought to kill him. 25 Then Zipporah took a sharp stone
and cut off the foreskin of her son and cast it at Moses’[a] feet, and
said, “Surely you are a husband of blood to me!” 26 So He let him go.
Then she said, “You are a husband of blood!”—because of the
circumcision. (Exodus 4:24-26)
Picturing this scene of marital stife, it's not difficult to see her returning to her father's house at this point, while Moses and Aaron continue on to Egypt.