It's happened before, but at present the scale and depth of the problem is unprecedented. They simply aren't broadcasting the withdrawals. In a way, the size of the problem is a good thing - cumulitively people have literally lost millions worth of USD from Poloniex in the last three days or so. It might be big enough now that regulators want to step in.

Just see how big the problem is: https://www.reddit.com/r/poloniex/

People have been talking class action. People have lost $25,000 transfers for months, with Poloniex keeping the change.

Wait and see, maybe it will resolve when after people start work in the US in about 6 hours.