Hedge funds were betting that the stock would go down. The done that by borowing shares of the stock from banks and selling that on the stockmarket, with the promise to the banks to return the stocks at a certain date.
At a certain point games said, fuck that shit and collectively buying massive amounts.
That forced the stock price up to a point where banks had to call the shorting brokers with 2 options. Deposit money to cover the risk of the high stockprice, or get out of the market by buying stock at the current insane prices.
At that point hedge funds tried to get out the short position (by buying stock) faster than competing hedgefunds, driving the price up to current level, extra fueled by people who still buy over 300 dollar because fuck it.