June 14th: Poloniex has realized something went wrong, and it starts to reimburse its customers. I have about 10% back, and in an email, Poloniex wrote that "You can expect additional payments in the future."
It's great, great relief, even if I'm still waiting for the remaining 90%.
I am guessing Poloniex sent some kind of demand to customers who have negative equity to repay the negative equity and some customers repaid, totaling about 180 btc. Poloniex possibly has some kind of legal reserve in case some customers sue it in relation to this incident, and this risk would be removed once they repay all their lenders, so they may not need customers to repay 100% of their negative equity.