I don’t know how well validated this is, I’ve only seen the same screenshot multiple times, no others. While if there is a margin call RH would have the right to sell down positions to meet the call, in this case it’s not clear if that’s the reason. It would also be problematic because they and many other brokers first manipulated the market by preventing purchases or the stock, causing it to tank, leading to them selling at a price below what it otherwise would have been.
It depends on the circumstance. Broadly, for straight equity position, they can close it if you owe them money (on other positions) or you haven't met your legal requirements (letting documents expire etc) or they think your abusing the T&Cs.
The fact this says "market volatility" is very suspect. That's not a reason to close my position unless I had a sell order in anyway but it went through with a bad price.
The fact its one screenshot from a twitter thread full of outrage about elites and 12 year old bailouts makes me suspect the user is just unhappy and there is likely good reason his position was closed.