Actually, Djokovic is more of the aggressor when he plays Nadal (both can counter-punch and go on the offensive of course). Usually Novak attacks while Nadal chases and counterpunches until he finds an opening. This dynamic was amplified in 2011 of course. It's just the MATCH-UP.
Novak just tends to become more conservative and passive when he plays Murray (for whatever reason).
RF (4-1-7-5-6-22-80), RN (1-9-2-2-0-27-64), ND (4-0-2-1-3-19-45), AM (0-0-1-1-0-9-28)