Actually, it's not a guess. Oftentimes when there is a virus definition file update, most of the anti-virus software thinks it's in their realm to reset everything on a user's PC. They don't ask, they just do it.
What I've learned over time is to take a screen shot of my allowed and banned sites and save it elsewhere. Inevitably, an update occurs, I get some funky stuff happening, and all I have to do is reset everything the way I had it.
Thank you, Bill Gates, for teaching the world that if you just force stuff down someone's throat, they have no choices and you can control the world.
25 years of IT and I'm a bit jaded, can you tell?