Well some of it is just good circumstance for Federer. Of their last 5 meetings (5-0 for Fed, Murray won 3 of 4 before that) 3 of them were in 2014 which was the year Murray was coming off his surgery. Federer did beat him at the AO coming off a lousy 2013 with his own back problems though not as severe as Murray's, but the 2 other meetings were in Cincy and the WTFs, and that was the year that Murray played a load of tennis just to make it to London (6-0 6-1 match). In 2015 they played on 2 of Federer's favourite courts and he played lights out both times.
All that said, it doesn't excuse Murray completely as he's been unable to win a set in a long while and he's even having trouble finding ONE break of serve in a match. The other major reason for the Federer turnaround in the H2H is that Federer is not getting stuck in as many baseline duels with Murray as was once the case. And he's not slicing back the Murray serve which by any account is below average for a top player. So now he attacks the serve more and gets to the net more. This works pretty well against Murray because his passing shots are not actually top notch. It might sound strange to say that, but they were never quite in the league of a Nadal either for example. Djokovic is better than him as well, but neither are in the class of Nadal.
Murray doesn't do particularly well when rushed or surprised. He likes to get into a baseline rhythm which Federer used to give to him when he still thought he could beat everybody from the baseline himself, but now that he's accepted that he needs to shorten points his tactics are working against Murray in particular, but obviously Nadal and Djokovic sometimes, along with the rest of the field as well.
