heya, same thing happened with me, the master cylinder for the clutch was never really adjusted properly by the mechanic who put it in when i first bought the car, i just kept up with the pedal coming nearly all the way out for like quite a while, i started to have probs where it just started slipping, but normally a clutch only slips a little, not rev out, so it didnt make sence.
i took a different approach and shortened the push rod on the master cylinder, and presto, the clutch works awesome, i have faster changes, and not one hint of slip. if u find this problem, dont cut the pushrod too much, maybe liek 5mm cuts then test each time, to get the desired feel from the pedal. then yep ur set.
bit of mucking around but itds well worth it, and may save u a costly bill at the mechanics if they decide just to chuck in a clutch even though it didnt need it at all.
as to why this happens, im not sure, maybe wear from the master/slave cylinder.