This is a phrase used in counter-strike or any type of PvP / FPS / RTS game. It is used to taunt your opponent when you have either landed an amazing head shot, double kill, or completely owned a bunch of noobs.
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: You, you're a hacker!
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??