Cause

1) Friction surface may be ground by worn facing fibers as abrasive compound, if you keep clutch slipping continuously i.e. hill start. It may cause judder because of too good surface to become mirror finish.
2) Clutch slave cylinder corroded or worn and piston not returning freely.

Solution

1) Driver education not to activate the clutch in an excessive semi disengagement position.
2) Apply proper amount of grease and replace worn out or scratched parts.