If you have a margin account, the broker calculates a risk based margin and can close any position they wish if they deem it a risk. They don't have to give you any justification. With increased buying power comes increased risk.
In some sense this was basically inevitable. Robinhood allowed (perhaps even encouraged) a huge mass of clueless investors to flood into the market. As long as the going was good, nobody complained, but as soon as things go south, people have started to learn expensive lessons.
This is not the first time this has happened. Almost every single market stress for the past 20 years has caused grief to retail in some way as they were caught out on something they didn't know about.
One clear example that comes to mind is when the Swiss national bank has removed the currency peg and a whole bunch of FX retail has made the expensive discovery that stop limit orders are not guaranteed to execute...