It's common in standing-room-only punk and rock shows. When the music starts, people push forward, so the crowd compresses shockingly tight. There is so much pressure that your arms are pinned to the person next to you, unless you raised them when the music started. Combine that with the oddball-variety of people who attend these shows, and you get conversations like the following:

   Ben: Brian, are you touching my ass?
   Brian: No.
   Ben: I was afraid of that.

I wondered the same thing. My first thought was "some gay clubs are like that", but I do not wish to make any unwelcome assumptions about anyone's sexuality :-)

If you're looking for extreme gropings, I'd say Ramrod's backroom in Boston is about as good as it gets. Just sayin.