After changing all belts, riding mower moves on its own

[TimS1959$]

No manuals available. Attached is the mower info.

 

[Tonebalone]

This might help:

https://www.troybilt.com/en_US/prior-year-models/bronco-42-troy-bilt-riding-lawn-mower/13WV78KS011.html

 

[StarTech]

Sorta sounds like a bent engine drive pulley that is grabbing the belt on every rev. This can happen during forcing on a belt. Doesn't take much either. Just to move the primary drive belt a little on each rev.