This seems pretty obvious to me, so I'm kind of scratching my head over why it's standing out so much for others.
Wearing a mask makes you somewhat anonymous. When you're anonymous, you have fewer inhibitions. People realize this and purposefully employ masks for this reason, as plainly evidenced by things like Carnival, Masquerades, and Halloween.
Heck, all of us posting here are doing so semi-anonymously with our own sorts of "masks". Sure, anyone who is interested enough can trace the identities of most of us down, but that's not the point (and it shouldn't be necessary except in noteable cases of abuse and whatnot).