Serpentine Belt Won't Turn on 98 Altima

[Spthomas]

My daughter was driving her 98 Altima, and it died. When we went to look at it, the engine would crank, but the serpentine belt didn't turn. I could see the crankshaft pulley turning, but it turned in a jerking motion, not the smooth turn I'd expect from the crank. Like maybe the starter clutch is slipping some because the crank is in a bind by the serpentine belt? Anyway, the crank would continue to turn, but the belt didn't. We tried this several times. Then, all the sudden, everything's turning, and it starts. Then sparks flew from the alternator. But then it all acted good, and we made it home. What can cause the serpentine belt to freeze up, but the cranks still trying to turn it (in a jerky motion, like the belt is holding it, but it turns some). Then it starts!