Term came to light on the well known tube video 'mad cause im stylin', where a rap battle took place and the guy who used the line got socked in the face.
Basically, it is used to anger your opponent and declare your dominance.
Noob: OMG HOW DID YOU GET ME THROUGH THE WALL LIKE THAT!!
Random guy: RAINDROPS!!!!
You: lol you mad cause I'm stylin on you??