Defaulting on bonds- If the shorts have put up bonds to cover the broker/clearing firm in lieu of cash margin, then they may need to put up more collateral(cash or bond) to cover increased risk now that the price is up. I would be surprised if the entities shorting this were small-frys. I understand that some of the algorithmic trading they were getting hit with when they were on the bid were quite sophisticated.
News 
Market Data 
Discover 
AI
