The collateral requirements depend on net buys vs sells. I'm guessing RH was tapped to the limit, and the buy limits were applied to anything with unbalanced orders. That applied to a bunch of stocks then, some that were up, and some that were down and looked cheap. (SBUX and AMD were among those down after earnings last week.)

whatever the reason it’s clear they can’t handle whatever volume they are seeing right now. who would continue to use them at this point.

most momentum trades are crowded - this implies any new crowded trade will have to be restricted.