Strange, I never had this issue yesterday when I was updating one of my Win 8.1 systems.
In my understanding, you have enabled a setting in the web shield (warn of low reputable downloads) which is not selected by default.
This will mean that anything new/unknown to avast! which has low prevalence and other factors such as digital signature to alert a popup.
So this is not a false positive since you have chosen to set up avast! this way.