That's an interesting perspective, I can certainly see why you think that would be a foul. At the same time, BBW rules are safety rules, and I don't think there's anything inherently dangerous about the action as described. Seeing it live or on film would likely make a difference as to how I see and assess it.
Also - the definition of a block: (2-3-1a): Blocking is obstructing an opponent by intentionally contacting the opponent with any part of the blocker’s body.
One could make the argument that (very) technically, laying down on the ground isn't a block, unless A somehow initiates contact with B. B crashing into, or falling over A, to me doesn't meet that requirement.