Once I played on a poker site. Don't remember the name. They had some 6 seat sit and go's. I was rather proficient at such tables, and they claimed that if you won 6 in a row, you would get a superbonus and win quite the substantional amount of money. I managed to win 5 in a row many times, however on the 6th attempt, there was always some kind of freak occurence that prevented me from winning. So I had 5 wins, then 2nd place several times. In the end I realized that it was indeed rigged, and probably there was no superbonus at all either.
It's extremely easy for a poker site to introduce subtle issues into their software that's hard to detect.
Classical examples of rigging is Las Vegas casions that have one-armed bandit machines near the entrance that have a high payout. So people get into the room, start playing and wins a lot, and they're thinking 'oh gee... it's my lucky day', so they proceed to other machines, or games and loses it all, which was the intention of the Casino in the first place.
Also if you run a poker site, there's nothing preventing you from looking at the hole cards of other players, and thus ripping players off in high stakes games for instance. The problem is however once a site get caught, it loses all credibility in the online poker community.
Anyway, the poker site makes money off comission, so if the operator is smart, he stays legit, he will still profit!