i guess they just burned out at an early age.
if i remember correctly, nadal, federer and roddick have been ranked in the top 10 for the last 3,4 years, luckily, i do not seem to remember any major events that occurred to those 3 particular players, and to this day, they are still going strong.
but, sometimes players just suffer lack of motivation, and after a long period of time, eventually they just lose the desire to go out on the court and fight it out for a win.
coria had his drug ban, i think it was a 2 year ban (cant remember correctly off the top of my head). gaudio (i think after getting bagelled by fed 6-0, 6-0 in TMC SF in 2005), he just was not really the same after that match.
just my 2 cents worth...