How do you call this?

[ranjo]

I have seen a similar thread in the past years, but can't find it now and can't remember how it ended.

A1 takes a short jumper from the free throw line. B1 turns toward the basket anticipating a possible rebound. The ball bounces off the backboard toward A1 who secures it and drives into the lane for a layup, but in the process contacts B1 from behind, knocking him to the floor as he makes the layup.

My problem is this: B1 did not have a legal guarding position as he was not facing his opponet, but he did get where he was first and he did get there legally.

Minor contact I would let go, but I'm not sure how to interpet the rules when there is excessive contact.

Block? Charge? No Call? How have some of you handled this?