who knows? I dunno if it has to do with the forehand specifically, but the way he's playing, he's having to work and run a lot harder to win matches, and that obviously causes injuries (see Exhibit A: Rafa). Plus Andy is just clumsy. Now they're saying he has changed his FH this season to hit it a little flatter. i did not notice the difference but ok if they say so
I think the commentators just like to talk about things. No one really knows why Andy changed up his game like he did. It's a mystery, even to those who follow tennis closely. It's just like one day he decided he was going to move away from the stuff that brought him all his success early in his career. It's not a loss of form/technique or anything like that; he didn't forgot how to hit the ball flat. It was a conscious decision made by Andy that he would out steady and outlast his opponents rather than take unnecessary risks.
Some experts like to say the power game has gone away and this switch was made to keep up with the competition. I disagree though, players are more powerful than ever with guys like Soderling, Tsonga, Del Potro, etc. I think it's been proven you can still win by hitting the ball hard and flat, over powering your opponent and going for winners. I don't know what goes on in Andy's head but for him to become relevant again it won't be a change in the forehand technique that will be required, it will require a change in mentality; from passive/defensive to aggressive/offensive.