Here is the customary interpretation: Moses had circumcised one of his sons, but not the other. This was because he was honoring an agreement he had made with his uncircumcised father-in-law, Jethro: Moses had consented to circumcise one child as an Israelite, but the other would remain an uncircumcised Egyptian. But now Moses had passed beyond the level of Jethro, and this ‘agreement’ was not acceptable to the Lord.
Others say that God belatedly wished to punish Moses for killing the Egyptian, or that God was still resentful at all the objections Moses had raised at the Burning Bush.
But none of these interpretations have a satisfactory ring to them when compared to the extraordinary bloodiness and eeriness of the story. There has to be more.
God’s attempt to kill Moses means that something in Moses, something in the Initiate, would have to ‘die’ and be reborn. Circumcision is a universal symbol of rebirth.