Here's an idea: Psychotophobia (fear of schizophrenia usually) is actually a specific/subset of or extreme agoraphobia:
"Agoraphobia is a condition where the sufferer becomes anxious in environments that are unfamiliar or where he or she perceives that they have little control." - ye good 'ole Wiki.
Derealization -> unfamiliar environment -> perceptively less control (depersonalization may exacerbate this through its tendency to disrupt the sense of executive involvement) -> anxiety -> fear of losing said executive authority (control) -> fear of psychosis / schizophrenia (for it is a condition where such can be the case).
Or, roughly so. I don't advocate the accuracy of this, nor do I believe it per se, but it popped in to mind just now, so I figured I'd share to see what others think. So... thoughts?