The Final Score

Psychology students can probably explain to me what the term is for this kind of behavior:
When I got off the subway at 161st St for tonight's Yankees game, there was a low buzzing sound going on and on because the emergency exit door was open. Now, even though there was a big red sign that said "Emergency Exit -- Do Not Open -- Alarm Will Sound" -- most people chose to go through the emergency exit rather than take the extra steps to the turnstile gate thing.
Full disclosure: I was one of them.
My analysis is that people are more apt to do the "wrong" thing once they see other people do it. And the more people who do it, the more people will continue to do it. Plus, the buzzing sound wasn't very loud -- certainly not enough of a deterrent.
Again, I'm sure there's a psychological or sociological term for this. Can anyone out there help me?
Thanks to speedysigns.com for the photo.