Poloniex got itself into trouble by operating unlicensed margin trading and lending services. This is most likely the reason why they are having issues honoring withdrawals because they're an illliquid exchange borrowing coin from Paul to pay Peter. They got in over their heads and the only way out was to pass the buck to Circle.
Well, I really don't know an exchange in production now that has the license to operate with margin trading and lending. Imho is not a valid reason to deny the withdrawals.
They auto-close the margin orders to avoid default