Maybe the belt is slipping (not tight enough)?
Maybe the drive wheels have worn down?

Good questions. I haven't opened the thing up yet. But yeah, I will
definitely poke around, call Yard Machines, and let y'all know.

It'd be a shame to let this mower go. The previous owner went into a
retirement home, and left us all these wonderfully maintained things.
The lawnmower just purrs like a kitten otherwise.