Could possibly be a exposed wire touching the frame causing it to shorting out, I would check the wiring harness and make sure there are no exposed wire that has been stripped and/or chewed off by an animalfuse blows only when PTO engaged
model # 13wr91at009 4456928So is it electric PTO or manual?
If it's electric it could be an internal short in the clutch causing the issue.
Give us the mowers model number, it will be located on a sticker on the bottom of the seat.