Well... headcases aren't mindless, so I'm not sure how well zombie tropes apply. They are just acting irrationally, with (to some extent) incomprehensible motives, often in a coordinated fashion which belies that they may not themselves understand -- or even be aware of -- their reasons for doing it. Some may even be unwilling passengers in their own bodies, "along for the ride", and begging for help to stop them.
Cultists in Lovecraft's works may be better models to draw from, less the ritualistic behavior... although it's hard to rule out even that, as they try to make sense of what's going on themselves.
In the opening scene of Shadowrun Chronicles, there are what can be presumed to be headcases performing surgery. If they are more "complete" headcases, it's entirely possible that more fragmented headcases may end up attempting (and probably failing) similar operations elsewhere, resulting in bloody messes in alleyways and squats elsewhere, where they started something and were forced to give up on in the middle... or against the odds, succeeding, somehow. This would, of course, be difficult to distinguish from various runners "investigating" the phenomena, despite a lack of understanding or the medical/technical skills to do it right. From the various commentaries in Stolen Souls, it's probably more common than most want to admit.
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