trading a zero sum game?

seems like it is to me. ive read many complaints about hedge funds and the "bear raids" that they pull off. take two hedge funds. hedge fund 1 executes a bunch of short sales on a thinly traded stock swamping buyers. it seems like a bigger hedge fund i.e. hedge fund 2 could simply start buyin the stock drivin up the price. i guess hedge funds could start colluding agreeing to stay out of each others stocks. dont know if such activity i.e. colluding is within the rules.