In round 1 you have more:
-Guys who aren't that used to playing best of 5. They have relatively poor stamina, they are able to win the first two, but don't have anything left.
-Slow starting 'big guys'. Haven't played a match yet, wake up late and then routine the nobody. (as Statracket said)
-Chokers. Pressure of the big stage gets to them.
Think this is the reason you see more of this in round 1 (when you're looking at the %, obviously there are more in total, but I think everyone will understand that)
Agree with this, lower ranked guys not used to play best of 5 - don´t have energy-stamina to win in in 3 than they are over - will loose
2. better players - loose beginning of the match, but still can come back and win this if they starts to play at least their standard level vs lower ranked guys.
that´s good one, but really there are players, who if fail to win in straight sets, just lost the momentum
i think there is 4th group and that´s - Davydenko is kind of that - yes he is old and bad stamina than few yeasr back, but this 4th group for myself are players they just play great for some period of time and than their level drop and it´s over - it has nothing to do with stamina or choke it , just you can´t play more than say 2hours- 2sets of high quality tennis.