I believe the statistics say money. However, I think usually the problem is a lack of communication *about* the money. Yeah, tough times are stressful, but it's when you start going at each other about it that it leads to divorce.
Even if you look at the forums here and what people come asking advice on - it can almost always be answered with "communicate with your partner."
My mom and dad are divorced for everything on there minus cheating. Though of course my dad is a paranoid person, so he'd say she cheated even though she didn't. O.o