Al there can be and is a major diference between a short sell and a buy.

With a short sell the seller borrows someone else's share and sells it, this in itself can create the illusion that the stock is going down.  Thus when the weak or those intended to belong feel they are losing profit often bail, causing at times a panic sell.  It is at this point the shorter looks to buy back the share to give it to the original owner. 

This senerio most often happens when a stock is in a narrow trading  range and awaiting the next news.  Buy short selling the shorts create a trend but it is a down trend, not good for a nervous long.