What's this Crud in my PS Fluid???

Last spring, while my car was in for routine service, the techs noticed the power steering fluid had become contaminated with crud. There was no
immediate cause found, so they changed the fluid, and I just put it out of my mind. Yesterday, the car went in for pre-winter service, and the power steering fluid was, again, found to be contaminated. Twice in such a short period of time? What could be causing that? Has anyone experienced a similar occurrence? As far as I can tell the reservoir is properly sealed. This has me mystified.
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