Probably heuristic method of detection. Maybe some code in Quests Extended looks like malicious and you get warning.
Anyway, there couple of anty-vir soft that giving false positives, my modpack was never infected yet, and I am always scanning it before releasing.
The announcement I posted I got at the moment I started WOT after the last modpack update
it says its a link to a infected file on the Internet.
This morning i did remove Quest Extended and i get rid of this announcement.