It's a bit of a known problem with the MS virus checker.
Anytime a new version of something is released, it can trigger false positives.
Dynomotion's KMotionCNC suffers the same problem until it's been out for a few days, and there's event software I use, which also has the same problem when a new update is released (MS even shut the app downand sandboxed it last month mid-event, which caused me a bit of a problem!).

It's all to do with the virus checker finding features in the software that could indicate it's doing things it shouldn't. In the case of the event software, it has a lot of functionality for replicating data to servers, which I guess gets flagged as potential Malware, and in KMotionCNC I suspect it's due to containing a lot of small example programs and source code, some of which are accessing system files. I'd imagine UCNC is being flagged for similar.