A common problem here with the deck belts coming off is the owner/operator removes the spindle pulley belt retainer cover because they are broken and just are too cheap to buy and install now covers.
View attachment 62082
I had one customer that just keep damaging the cover and removing them then he would bring the mower complaining the deck belt would not stay on. I no longer work on his mowers after it took 6 months for him to pay the last repair bill; At least I had his mower as collateral.