Short selling doesn't cause a stock's price to go down; it allows you to profit if it does. Basically, if I want to short a stock, I borrow it from you and agree to give it back on a certain day (paying some fee for the privilege). I then sell the stock. If it goes down, I can buy it back for less money, return it to you, and pocket the difference. If it goes up, I still have to return the stock to you, so I'm out the difference.

There are ways of manipulating stock prices, but they're generally illegal and short-selling isn't really one of them.