Rumor has it that Brad Pitt was working on a movie with Vince Vaughn about two years ago, and Jen would always be hanging out on by the trailer with Vince, and going off alone with him.
Anyways.. about 6 months later Brad started filming with angelina, and rumor has it that Brad was jealous of Jen and Vince.. in addition to the fact that Brad had been wanting babies for years, and Jen said wait till friends is over.. well then after friends was over for two years still no baby. According to US Weekly, those two causes influenced Brad to leave her.
Who knows whats true.. but its an interesting story.
I honestly don't think Brad cheated.. he may have flirted and developed a crush (for all we know he could have been mentally checked out of his relationship for months by then), but I think based on what Angie has said, and the fact that Brad's such a family man, they waited until the divorce was filed.