Defender intentionally falls onto hands and knees...

[PSidbury]

12-14 yr old boys summer league using NFHS Rules (with a few modifications):

A fairly large A-player has been establishing position on the block and with ball "backing-down" B-defender with regularity. B-defender is not flopping nor is allowing himself to be run over, but is at times is being forcefully pushed-off/bounced-off his position as A-player backs-up toward hoop.

At half time B-coach complains about the A-player aggressive backing-down, pushing his B-defender off his "position". I basically shrug and say I will keep an eye out for anything worth of an offensive foul.
I mention this to my crewmate, but he just brushes it off as more or less some acceptable hard bumping and the smaller B-player not allowing himself to be run over.

So, on the first A-possession, sure enough the ball goes in to the A-forward and he feels the B-player on his back, so he begins to back down.

However… the B-defender immediately drops to his hands and knees (within his space) and… you guessed it… allows the A-player to tumble over him.

It was a two-man crew and I was trail, so lead official immediately calls the foul… on kneeling B-player.

B-coach goes irate; wanting a travel called on A-player. Lead official says to the coach, "Your player tripped him, coach, and not only that it was unsportsmanlike and intentional."

I mean, part of me equates this to the classic Kevin McHale "pulling the chair out from underneath" maneuver, but on the other hand falling to one’s hands and knees does seem like crossing the line into intentional unsportsmanlike conduct.

Is something like this in the NFHS Rules/casebook?

What if the player decided to get on his hands and knees away from the ball or other players?

What would you guys have called?

Thanks,

P.S. Crewmate says to me after the game, "I'm pretty sure the coach told the kid to do that."