Help with an airborne interception

[ChuckS]

A1 passing to A2 in the FC, B1 jumps and intercepts the pass. He landed on stationary A3. I called a foul on B1. Coach complained that he was making a play. I said that doesn't give him the right to crash into another player.

But later I thought what if A3 was still moving after B1 became airborne, then the foul should have been on A3.

Any tips on officiating a play like this? Clearly I needed to be looking at a lot of things at once, but I did not notice if A3 was moving when B1 became airborne.