[CoachNRef]

Originally Posted by BktBallRef

No, that's not "just the way it happened."

Did B1 have legal guarding position?
Did B1 fall down while backing up?
Did B1 move toward A1 as he fell?
Did B1 fall into A1's path as he tried to go by him?

If B1 had not fallen down and the same collision had occurred, would there have been a foul? Who would it have been on?

So no, you did not give us enough information to know whether a foul occurred or not.

yes, yes, no, no

if B1 hadn't fallen down, you could not have the same collision. now if B1 stopped would A1 have run into him? maybe.