Don't Hit Me!!!

[ABC Coach]

I reffed a game last night and saw a play I don't think I've ever seen before on any level. A1 starts to drive the lane. B1 has perfect position to take the charge (he was standing about 4 ft away from the hoop). As A1 approaches, B1 starts backpedaling. A1 finished his shot (missed it) and ended up in B1's lap on the floor. I called it a block because B1 obviously didn't have his feet set. That being said, he did have legal guarding position before the contact because he was at the spot first (and the contact happened torso-to-torso).

I don't know what made the guy backpedal like that. I've seen guys turn away just to shield themselves from the hit they're about to take...but to start backpedaling was a new one for me. Did I make the right call?