The SC bearings are pretty robust and submurged in oil. The pulleys use sealed bearings designed to be permanently lubricated. The trouble is that they're out in the engine compartment and exposed to heat cycles and road debris.
Point being, they're definitely the weak link. The rubber seals will crack and fail - allowing the lubricant to escape and the bearings to run dry.
About 3 years ago (80k miles maybe), one of my pulleys failed and tossed a belt. While I was at it, I took off all of the pulleys and noted that they were all failing - the bearings were dry as a bone.
Take your belts off and remove each pulley. My guess is that your bearings are also failing. If you spin the pulley by hand, you shouldn't hear anything and should have a slight resistance. If you spin it and it spins freely and you can hear the balls rolling against the race, then the bearing has failed and the pulley needs to be replaced.
