Imagine that someone is borrows 5 bananas to sell, under the impression that the price of bananas will go down, so they will ultimately make a profit when they return 5 bananas to the lender. But then others buy ALL the bananas, and make the value of bananas go up ridiculously, like into the thousands. This becomes a problem for the guy who borrowed bananas, because he has to buy bananas from the guys charging thousands for them in order to pay back his 5 bananas.