Rx75 30" riding mower

I bought a used rx75 30" rider and needed a new blade belt. The old one was bad shape and had a groove cut on the flat side of the belt. Not normal! I put a new JD belt on and within minutes the same groove was cut on new belt! I think it has something to do with the blade engage/disengage mechanism. Belt still works and groove isn't getting deeper.

Can someone shed some light on what my problem may be?

Thanks,

Deerejohn