It's not, they don't give any reasons in that blog post.

They vaguely hint at things that could be the reason, but they never actually say those are the reason. This is either shady or horrible PR.

That is not a source for what OP is saying has happened.

> As a brokerage firm, we have many financial requirements, including SEC net capital obligations and clearinghouse deposits. Some of these requirements fluctuate based on volatility in the markets and can be substantial in the current environment. These requirements exist to protect investors and the markets and we take our responsibilities to comply with them seriously, including through the measures we have taken today.