It's a short squeeze:
> But if a stock rises steadily above the price at which the short-sellers initially sold it, they are sitting on a loss. That loss — in theory — has no limits because a stock can keep rising. And if a stock zooms higher, as Tesla’s has, the short-sellers will usually have to rush to buy the shares to protect themselves against further losses. If enough investors do this, it pushes the stock price up even further, forcing even more buying by short-sellers. Other investors often join the buying, in the belief they can make quick and easy profits.
