Well, they've published a blog post. And got CEO on the news. But what they offered was not an explanation.
The explanation they offered in their blog post:
"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."
That's not an explanation, that's just hot air.
What financial requirements? What capital obligations and clearinghouse deposits? Which requirements fluctuate based on volatility? How do these requirements protect the investors and the markets? And ultimately, how exactly any of this leads to them blocking buy orders on $GME?