Why do guys like grabbing girls butts when dancing, so much?

So when ever I go clubbing a start dancing with a guy they ALWAYS go for my butt and they will do it to other girls. Its as if to their hands just magically slip there. The other day I went out clubbing with work and a engage Co worker of mine was being very aggressive whilst dancing with me, he was pulling me around like a rag doll and being very grabby. My sisters were also with me and said he was the same with them and he wouldn't stop grabbing are butt's. I tried to move his hands away but he'd just do it again, he was such a creep when at work he extremely shy and awkward.
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+1 y
It was a ghoth club by the way, full of lots of head bangers and screamo music
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+1 y
I wasn't even dancing with him at the time he did it, I was dancing with my sister and he kept grabbing at us and trying to hug
Why do guys like grabbing girls butts when dancing, so much?
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