People relate a bad alternator clutch pulley or a bad tensioner pulley to be the source of most jumping belts. Use the SEARCH feature.
I replaced my alternator ($180 200 amp Bosch OEM) after hearing a "chirp" when shutting down the engine. Belt was jumpy, less jumpy after replacement. I decided that a new alternator with the clutch installed was a better solution than saving $75 and putting a new clutch on the 8 year old alternator; and I didn't have to buy the special tool to remove the pulley.
The belt was still a little jumpy, so I checked the tensioner and found it to be lacking. Replaced it and the belt is now super smooth. The tension on the new one vs the old one was significant. I think your tensioner may be to blame, or both working together to throw the belt.