They're likely close to hitting their collateral limits with the clearing house[0]. During yesterday's 5-minute interview with CNN's Chris Cuomo, RH's CEO mentioned their collateral obligations (plus unnamed regulatory requirements) as the reason for preventing new positions in GME. Every trade creates a collateral obligation with the clearing house for 2 days, so if RH goes bankrupt while the trades are being settled, nobody loses. The amount of this collateral obligation is a function of the volatility of the stock. Lots of high-volatility trades in the middle of the 2-day settlement process eat up collateral very quickly. If RH runs out of collateral, none of their customers can trade until some of the pending trades finish clearing.
The reason for still allowing people to close positions is presumably because people would scream bloody murder even harder if they were suddenly unable to exit these high-volatility positions.
Collusion to manipulate markets, particularly in such an incredibly high-profile situation, seems unlikely. Whatever RH does, they know there will likely be investigations, so they're almost certainly on their best behavior right now.
