Why does my belt fly off when I engage the blades?

After having pulleys replaced by a Sears certified repairman, the belt that engages the blades periodically flies off and tangles, slicing the belt, when I attempt to engage the blades. About every third mow. The repairman claims it’s my fault because I don’t clean under the deck every time I mow. He is incorrect. I have gone through seven belts in two years.