Snape was dragged into a party in the room of requirements. Don't ask how he just was. There were alcohol, music and a huge variety of people. There were groups of girls huddled in groups dancing, people grinding on each other, and some groups of guys simply drinking. Aka this was not really Snape's kind of thing. He was about to leave before he saw something that caught his eye...

You drunkenly dancing and you were GETTING IT.