Why is jamaican dancing so violent?

There is actually a lot of different Jamaican dance styles, some of the more recent ones have become fairly aggressive.

That form of dance style that you're referring to is called Daggering which is a recent dance category started in the early 2000's. Daggering is where a man and woman are dancing in a sexually provocative manner (dry humping), Which usually includes them simulating various sexual positions in an aggressive manner.