Why would a girl pinch my butt and walk away?

It was weird I was talking to an old friend of mine and and I felt someone pinch my butt. It got my attention and when I turned around I saw three blonde girls walking away. I didn't see who it was so I asked him and he said it was one of those girls. I didn't know how to handle it.
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Why would a girl pinch my butt and walk away?
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