Well, a lot of cases it's about wanting your fifteen minutes of fame.
I mean, look at that couple on Amazing Race. I think his name was John. They were supposed to be entrepreunerers. He was constantly screaming and yelling at his wife. I think she did her fair share of yelling and screaming back at him. They also went on Fear Factor and he was acting the same way.
When someone puts a camera on you, you stop acting like yourself and play to it. Look at Judge Ito and Marsha Clark in the OJ trial. It's not normal to have a camera trained on you 24/7. Knowing that one is is going to make you act in ways you don't normally do.
BB is probably the worst reality show in regards to the cameras. They know there's people watching them on the live feeds. They can't even go to the bathroom without being watched. Under those conditions it's impossible to behave like yourself. Amazing Race only films people during the race. They can safely go to the toilet without knowing someone is watching them.
It goes down to how one handles the psychological stress of being put in a situation like that. Look at what happens when a disaster occurs. While it brings out the best in some, it also brings out the worse in others.