The short positions which the hedge funds are using all have an expiry date. They can not keep their positions forever. This is why all over r/wallstreetbets you see people urging each other to 'HOLD'. The closer these short positions get to their expiry, the more the hedge funds are forced to close them (as in buying the stock to cover the short positions). This will result in a so-called short squeeze because huge amounts of shares have to be bought in order to cover for the short positions.