Actually, nobody's totally sure of what happened.
We know what Poloniex said, but a guy I exchanged with on Telegram said he has proof it was a theft.
We shall remember that it took Poloniex 10 full days to react, and explain what happened. Why did it take so long?
I assume they were in damage control mode, figuring out their legal options and obligations, deciding whether and how to socialize the losses, etc.
Did you ever see this supposed proof? I would take it with a grain of salt if not. I don't believe there was a corresponding hot wallet withdrawal at the time, or any other signs that it was related to a hack.
Poloniex at first said it was fully commited to reimburse anyone, and then nothing for over a year.
That's Circle for you. They've been all over the place for years. I think they had high hopes for Poloniex, then saw that volume completely died after the 2017 bubble popped, and eventually decided they wanted nothing to do with the altcoin exchange business. I think this margin lending disaster was just another thing pushing them to get out.