You say it has two v-grooves, plus a serpentine in front. How deep is the pulley, what are the different belts driving, and how are they tensioned? I assume the serpentine is kept tight via a spring loaded tensioning pulley? What about the belts in the v-grooves? It sounds like the pulley has been flexing with rotation and fatigued around the bolts, and too much belt tension would be a possibility, especially on a deep pulley - lots of lateral force pulling on the belts. Just thinking aloud - nothing else makes much sense.