Funny how that looks in hindsight. He went on to complete his Career Slam at Roland Garros in May and then break Pete's Slam record at Wimbledon the same year. I think the pressure of chasing the Slam record really got to him during this period.
I've rewatched the match highlights a few weeks ago. Federer was playing like sh*t in what I think were windy conditions. Missing a lot of easy shots, so it's not like this one miss caused him to do it, was more of a build up across the match. Djokovic was obviously playing well, but it just wasn't working for Roger that day.
In many ways it was a buildup of the entirety of 2008/2009, losing the AO final, Wimbledon, WTF 2008, and generally not dealing with the pressure of chasing the Grand Slam record gracefully.
Anyways I actually enjoy this Federer moment quite a lot. He actually showed emotions on court and was not just a polished media personality. Humanity > Spokesmen.