This is it. Everybody accusing Murray of faking should read this post first.
Regardless of whether it's a tic or whatever, he was clearly only clutching his back after losing points, so much so that it became hilarious. He did it through the first two sets, including that final game of set two where it got to a point where he was literally alternating between sprinting around the court and fistpumping the points he won, then leaning over and holding his back while grimacing on the points he lost. There was not a peep from him in set 3, but then when Gasquet tried the shock 'n' awe tactics at the beginning of set 4 and he lost that deuce point, he clutched it again.
The RG crowd are generally disgusting, but he brought all the booing on himself.