It's the domino effect. It starts with how well he's feeling physically (not to say he's injured, but he's had a injury-filled year and is obviously not in peak shape as he bids to put them behind him), then you have the fact that he is suffering one bad defeat after the other (starting with Wimbledon). As a result, it's going to affect his confidence and the way he's playing, which usually affects the shots you're the least comfortable with (so the serve would be up there, especially since it hasn't looked good since the back issues). When the serve is gone, the forehand just isn't going to look that good since he's not always hitting the shot from an ideal position, and he's going to be reluctant to go for it as the rally progresses. One bad thing leads to another.
On the other hand, it really doesn't take much to go from that to playing relatively well again. It's not like he has to fix these issues at length individually. If he loosens up, goes after his forehand, feels comfortable in the rallies, then he might feel confident in going after the serve as the match progresses, or the other way around.