Keep a heads up when around that type machine when off the machine with it running.
It's the type that the blades will start turning with the deck lever all the way up. Why As the belt sets in one place on the engine pulley it gets warm/hot and friction increases and the blades start turning. It's a common issue with that type, especially if it does not have any deck brakes and will even do so with deck pulley friction brakes and it only depends on the slack in the belt to not rotate the blades. The length of the belt is very critical for temporary dis-engaugement by just slacking the belt. operate the deck up down with the engine not running, as you raise the deck it should move forward and slack the belt and as you lower the deck it should move back and put tension on the belt. (You are playing with a poor design but they will cut grass good when all is correct. (just don't trust the deck blades remaining in neutral when they should)
As the deck is lowered the deck moves back and tightens the belt. You need to make sure the belt is correct length and then go through ALL the deck adjustments. The front of the deck is too low now.
If you check all the deck adjustments per specs and the belt is correct and it still has issues remove the deck and check the bearings and make sure a spindle leg is not broken or the deck rusted out or bent under a spindles.(that allows a spindle to lean forward under tension)
You can check the length of the belt using a $4 cloth upholstery tape measure. I've seen belts bee off as much as 1inch in length vs what is stamped on a new belt.