I think the Juventus loss really was the final event that tipped the decision, although I believe the Chelsea board wanted to get rid of him for some time now.
Juventus is a really good team, #1 in Seria A if I'm correct, which really annoys me because my favorite team in Seria A is AC Milan, which hasn't been good since the 2007-2008 season. Back on topic though, I don't blame RDM for losing to Juventus, I think that they are much better than Chelsea and probably should have won, regardless of who was manager.
For me, I'm really starting to get a kind of ungrateful-ish vibe from Chelsea. I believe RDM used to play for Chelsea. That's not cool, that's like if Kobe Bryant became coach for the Lakers and then got fired :/