Why would a belt switch burn up?

Why would a belt switch burn up?

I turned on the dryer and heard a pop and then it released a burning smell. I did my research and thought it could have been the belt, maybe the thermal fuse but turned out all of those were good and my belt switch was half melted and toast. Just want to know what causes that so when I replace it won’t happen again. The dryer was pretty dirty inside as well.

ANSWERS: