Because antivirus software tends to be a huge load of crap that will flag anything just slightly out of the ordinary with annoying popups to give moronic customers the impression their wasted money actually contributed to system security. Don't worry about it and tell users to remove that garbage from their systems.
If more developers stopped putting up with it for fear of being associated with malware, this shit would come to an end quickly.
Alternatively, you can bend over and ask them nicely to consider the flagging of your app a false positive. If you're lucky, they might add an ultra-specific whitelist entry so that their software will shut up until you update your app (or compiler, …) the next time. Repeat until your butt can't take it anymore.
[1]: https://trello.com/c/dFLnJXLr/121-mcafee-complains-that-fman...