Foul? Travel?

[ChuckElias]

Here's a real life play that we've discussed in theory. What are your thoughts?

A1 attempts a try near A's basket. There is minor contact as B1 tries to draw the charge. No foul call. B1 ends up on the floor near the basket just as A2 secures the rebound. One of A2's feet lands on B1 and A2 trips, going to the ground while holding the ball.

This happened in my Juco game today. What's the call? Foul on B1? Or travel on A2?

(I think I got it wrong. )