The other thing to note is that when you short a stock, as
@Heraclitus mentioned, you are borrowing that stock with the intention of selling for a profit once the share price decreases, i.e. your gain or profit results from a decrease in the stock price in the future. It's simply the inverse of a normal stock purchase.
However, when you short a stock your liability is (potentially) infinite, that is, so long as the stock rises you are on the hook to make up the spread if/when you want to reduce or eliminate your position. If you don't, as the borrower, you will be called on your loss, meaning you have to cover the interest on that difference, which is huge, or attempt to exit. Eventually, and this is what the 'activist investors' are betting on, is that there will be a short-squeeze. That is when those who hold the shorts are forced to buy up remaining stock in order to put a ceiling on their losses, but in doing so it will massively reduce/eliminate remaining liquidity (available shares) which will send the stock price soaring. It's a nightmare situation for a hedge fund or any investor going short, as they will eventually be forced to cover their positions at an enormous loss.
So, all in all, these funds are sweating now b/c they are out billions (on paper) unless the price drops a lot, which does not appear to be happening.