That's because you didn't hold the hex on the pulley. As I stressed in my previous post, the removal of that nut is very difficult without an impact wrench.
The only thing that's going to warp the pulley is something hitting it. It will not warp in normal operation. You will have to determine whether the pulley is warped, loose on the spindle (unlikely), the bearing/spindle assembly is loose in the cups (very likely) or if the bearing is worn out. Before you take it all apart you can wiggle the pulley and see if the whole spindle has play and determine what is going on.
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