Shorts borrow the stock for a time period. Let say you borrowed a stock that is $1 from me with the promise you will give the stock back end of the week + some interest. You sell the stock for $1 end of the week the price is $100 so you are on the hook for stock + interest. As the owner of the stock I will be more interested in getting it back and selling it for $100 rather than allowing you to give me the stock later by extending the loan terms when the price might drop.