Comment by r2d2-c3po

They wouldn't have to buy it back immediately. Short sellers would borrow the stock from a long and pay a predetermined interest rate for however long they're borrowing it. They technically don't have to rebuy the stock unless they get margin called (if the collateral posted is no longer deemed sufficient for the counterparty to allow the borrowing position) or need to close out positions because of some risk tolerance threshold being met. Usually when this happens to a bunch of shorts at the same time they must all rebuy, driving prices higher, which is called a short squeeze, but it is usually determined by someone cutting losses on a short position or positive news coming out driving prices up.