foul?

[Troward]

Player A1 is on a breakaway and relseases the ball for a lay up. Player B1 is coming diagnoly at A1 from the opposite side of the lane to try to make a defensive play but he is too late. The players momentum from running carries them into each other and they collide under the basket - after the release of the ball. IS this a foul? Any tips to determine at what exact moment does the continuous motion of the shot end?
When I am in "button hook" position on the opposite side of the shot, I seem to no call this a lot, but a lot of officials seem to call it in the act of shooting.
Might the trail at half court, but on the side of the play have a better view of this play?

thanks for sharing your thoughts
GTW