You can't determine a cause simply from correlation. Even if it were true that polygamous relationships have more abuse, that doesn't mean the polygamy itself causes it. Like owen said before, it's not polygamy causing the problems within the Mormon sects, it's the Mormon's.
It's like if you see a study that shows children who watch more violent TV tend to be more agressive, and then you conclude that the violent TV causes the behavior. It's possible that's true, but it's also just as likely that agressive children like to watch more violent TV, and that the TV doesn't cause the behavior at all.
The same is true in this case. There might tend to be more abuse, incest, etc., with polygamous couples, but that in no way begins to show a causual link.
