I'm going to risk disagreeing with Belarusian: as noted, we do not know all the reasons why files transferred to field offices rather than handled from eg. Mississauga.
My own view (informed guess) is that it's NOT just related to red flags, but likely any matter that either requires field office to follow-up or check or handle, or for which field office is better equipped to handle. (Likely other reasons as well that could be as mundane as 'which offices have spare capacity')
Medical furtherance MAY be a factor that a field office is better equipped to handle - but maybe not - including such factors as local medical system, presence of a regional medical office, or a bunch of other factors.
Also note: it's not just 'luck' whether field office gets the file, so there's no point getting stressed about it. Medical furtherance will at any rate likely take longer for processing (although likely depends on nature of the medical issues, whether treatment or follow-up or whatever else is required).