It was at the start of the season where several players aren't match-fit yet and before Novak/Muzza were that good and when Rafa was still inconsistent off clay.
Or these players were simply playing excellent tennis in their own right.
These runs by unexpected players are amongst my fonder memories in the sport. They acquired something which today's underdogs could only dream of. Balls, obviously.
People have bitter memories of Clement reaching the final, because seemingly almost everyone wanted and expected Grosjean to do so, but it doesn't take away from the fact that he strolled past a young Federer, beat former champion Kafelnikov and edged out Grosjean himself in an extremely close match, playing creative tennis and daring to approach the net, especially for a smaller man.
Schuettler was a rock, if not fortunate of the withdrawal of Safin and the fact Roddick was playing at 75% due to his marathon against El Aynaoui. He played fearless tennis up until the final where the wheels fell off.
Baghdatis played magical tennis. There was just a sense of inevitability that he would reach the final and if it weren't for his legs, I think Federer would have had to have played one of the most clutch matches of his career to win.
Gonzalez and Tsonga hit the form of their careers. Both served well and dominated from the baseline, with Tsonga's drop volley's never better.
People can say that it was because Nadal, Djokovic and Murray hadn't yet reached their prime, without judging the performances on their own merit, because either they can't remember them nor care to see them but every one of these players played a level of tennis worthy of a grand slam finalist.
The answer to your question is that nobody exactly knows why, each of these men probably have different reasons. What they all have in common is that they were physically and mentally prepared. Without the grind and troubles of the tour, I'd imagine that these players physical and mental bruises would have healed and that they come back onto the tour feeling refreshed and looking forward to the first big one of the year. There's no doubt that these players proved mentally stronger during that fortnight than any in their career so I'd be more inclined to go with the inkling that their mentally approach had been resynthesized.