What ruined Kate Winslet and Sam Mendes' marriage?

Revolutionary Road, starring Kate and directed by Sam, depicting a disintegrating marriage. Tension on set became tension at home. It's a persuasive explanation, repeated in two celebrity outlets, but maybe too pat. (Especially since Kate won a Golden Globe for that movie! That's the opposite of a mood-killer.) Until one of their nannies sells a story to the tabloids, I'm not biting. [via]