I think maybe his compression is upping from slowly wearing in. He says the clutch let out (letting go of the clutch lever) makes it skid (which only happens if you are already moving fast enough to skid aka: squeezing the clutch works fine people)
There are too possibilities, the motor broke in and is harder than normal to turn over, which I doubt is the problem in this case because of how sudden it was, or he has something bound up in the motor between the piston and clutch plates.
It is probably a seized piston or a failure in the clutch gear (right side if sitting on bike) case area.
I would recommend pulling the largest cover, the one on the right, and see if there is an obstruction in the gears, if not then it's the piston. Maybe a lean condition the op was not aware of and the engine just toasted itself while he was enjoying the ride.