Falling before contact

[SC Official]

We've debated this before on here, but here's the play that I had last night and want to hear thoughts...

NFHS: Secondary defender B1 establishes LGP under the basket. Before being contacted by ball handler A1, B1 begins to fall backwards such that he does not "take" the contact and that A1 doesn't initiate any contact.

We know that a defender can turn or duck to absorb the shock of imminent contact, but in my judgment that's not what B1 was doing. I ruled a blocking foul. However, I'm not convinced that the rules necessarily support my decision even though I've seen this play adjudicated identically in other games (NCAA and NFHS). My justification is that B1's action put airborne A1 in a dangerous position returning to the floor.

Is there anything in the rules that supports a blocking foul on this play?