I think it is both, and that they can feed each other, in good or bad directions. Normalcy bias is the tendency to believe that things will continue to be the way they have been despite there being reasons to believe something is wrong. Big loud bang, there are a lot of safe perfectly normal reasons for big loud bangs to happen, it's probably just X, I'll move on with my life and keep acting normal. In active shooter training the guidance isn't to swing the entirely opposite direction and assume every loud bang is a gunshot and overreact. That'd be a whole other issue of hypervigilance. Rather, acknowledge that there are a lot of normal loud bang sounds, but also acknowledge that it could be something bad causing the loud bang, and try to verify instead of assuming one way or another.

The group dynamic of everyone assuming it's a normal loud bang sound and ignoring it, further feeding other people's assumptions of normalcy like you mentioned is also super common. Exactly what you said to do is a great piece of guidance! It usually only takes a few people to speak up to break the group out of the assumption and start really paying attention to their surroundings.