game plat situation #2

[crew]

here is the setup.
you are slot opposite the table. a1 drives from trails area to the basket(with the ball) slightly out of control. as he drives he stumbles, throws up a shot(misses) and falls to the floor. b1, on backside rebounding jumps(slightly forward) and grabs the rebound. a1's momentum takes him under b1's feet. as b1 comes down from the jump he lands on top of a1 and falls to the floor with control of the ball. what would you call?

i called tripping(triping) foul on a1.
why? many thoughts went through my head.
1. my partners had no foul on the drive nor shot.(a1 falls without help)
2. a1 had no legal position laying on the floor.
3. verticality goes from the as high as the ceiling and as low as the floor.

i have asked many officials what they would call and get a 50/50 response. travel/foul
is there anything i am not considering that i should/could consider?