That's my thinking, but ignition is a staggered 6 second duration, so there's at least a window for adverse directional flow.

But looking at that pad and the damage to it- without deluge dampening, I bet the RC spalling might have been a bigger issue than just ground blowback.

Put a blowtorch to some concrete a and watch how quickly trapped moisture starts causing miniature explosions and flying shrapnel.

But that's just my first uninformed impression.