When you and a a couple of your close friends, sometimes more, drift away from the actual party to do 'other things'...aka drugs, in the bathroom. Therefor starting your own separate party in a small little fancy room next to some strangers toilet.
I mean you've got running water, nice lighting, rolled up 20's. That's enough for a nice little soiree
*BANG BANG BANG BANG BANG*
Stranger: What the fuck is that banging noise coming from the bathroom?
Bathroom attendees: Don't worry, Bathroom Party, we're just nailing up picture frames *sniff* :D