If I had to guess, it's because WriteRush hasn't been installed enough times for AVG to allowlist it and mark it as safe.
Other than that, I'm not sure what would cause the false positive. The desktop applications are built using Web2Executable (which uses NW.js). These are both open-source. The desktop versions use the same code as the web version, which you can find on GitHub and itch.
In the meantime, you can use WriteRush by downloading the "WriteRush v3.8 (ALL).zip" version, which runs in your browser. Also, I'll probably change the name of it "WEB" in the next update, because that's more intuitive than "ALL".
Thanks for bringing this to my attention, and happy writing!
— Kurtis Kumar