Excessive pressure (generally due to a buildup of debris in the system) or fine debris in the fluid; I think the belt would have to be extremely tight to cause these failures repeatedly and in such a short period of time. I'd flush and rebuild the rack and add an oil filter just before the PS fluid cooler.
I would also buy my own pump from another vendor for them to install, just in case they were trying to make extra money by just replacing the seal and skipping replacing the whole unit. As you were going to the same dealership for all three previously though and are now going somewhere else it's probably not a relevant concern.
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