Power steering pulley out of alignment

A few weeks ago, the serpentine belt jumped off.

I replaced both the serpentine and air conditioner belt. When I started the engine, the belt walked back about one rib, riding up onto the middle flange on the crank pulley. Upon further inspection, it appears that the power steering pulley is a few millimeters farther back than the rest of the pulleys that is, it is closer to the block, and not in the same plane as the other pulleys. I believe this is what is causing the belt to move.

Out of desperation, I sprayed some silicone lubricant onto the water pump "pulley", re-positioned the belt, and it stayed in place for a day or so. I believe this lets the belt slip on the (smooth) water pump pulley, compensating for the alignment issue.

My questions are:

1. How can I correct the alignment of the pulley? I can remove and reinstall it easily, but, without some sort of spacer behind the pulley, I will just wind up back where I started. Installing a spacer seems ill-advised, and it is not something I have ever had to do on another vehicle.

2. We bought this car over a year ago, and never had a problem until very recently. That implies (to me) that this is a recent change. What would cause a pulley to shift? How can it shift? The pump is securely bolted to the bracket, and the bracket is securely bolted to the engine. How does this happen?