Throwing a belt has a lot of different causes. Alignment of the pulleys is critical. (They should rotate as nearly as possible on the same plane, sometimes not possible on a riding mower because the deck moves up and down.) Pulley guards matter; if one is missing, that could cause the problem. Keep in mind that spindle guards, the shields over the outboard pulleys on the deck, often do double duty -- they protect your feet, buty they're also there to keep the belts from flying off. If one is missing, that could be the problem. Even a loose pivot bolt on an idler pulley can do it. A stick or other debris lodged under the tractor can do it.
Amazon just brought me a new set of drive belts for a Huskee Supreme LT I inherited. Now I have to put the dang things on it. I hate belts.