Clutch wearing around the edges

[Elect666]

While I have my motor out I figure I would of course change the clutch and plug off the two holes (just removed the air pump rod) on the back of the motor.I didn't pay much attention to the clutch when I removed it, just the outer edge of the clutch plate was worn, What do you all think would cause this? I am changing the whole assembly I am just wondering.
Thanks Elect666