Roddick letting Gasquet beat him at Wimbledon that year...
His play style takes some blame, I think. He plays too far behind the baseline, but doesn't have the tool to be a proper grinder or a retriever. Instead, he simply facilitates his opponents' efforts to step in and push him around.
Add that to the fact that he's not very physically fit, and mentally, he doesn't show much fight.
By the time he's in the R16, most players are going to comfortably be able to take it to him.