When you “go long,” your maximum possible loss is 100%, or your entire initial investment. That can happen, for example, if a company goes bankrupt.
But if you have a short position, there’s no limit to how much money you can lose if the shares rise. If the share price increases soon after you place a short position, you could quickly “cover” by buying back the shares and returning them to the investor you borrowed them from. If you’re lucky, you might not lose very much.