Probably one of two things.
The easiest fix is just to regrease the inner motor gears. With that much distance & time put into it, it's due for a servicing. The type of grease can be found in the main BBS thread. You'll have to browse through it to find it.
The harder fix is that clutch has either degraded or rusted. That requires some key tools and a new clutch. There are takedown videos of the BBS where you can deduce how to do this if this is the case, also located in that forementioned thread.
My instincts point to the first, only because in my case the clutch slowly degraded over time with more and more useage.
Edit: before doing this, make sure it isn't something as simple as your drivetrain & brakes aren't causing friction issues.
Surly Ogre rigid 29er, rear 10T MAC @ 50V 25AH & 40A: 30mph road/gravel/hill machine
42" dual diagonal Eskateboard @6s & 90mm wheels
Next: eMTB @10-12s & 8"-12" pneumatic wheels; Got Strapped? d-(',')z