Check this out...can i know what is this plz?

hey there....umm....iam a 20 years old guy and i have this habit...i have this involuntary shaking of my leg especially the right leg while iam sitting....when i sit down then i start shaking my leg rapidly and involuntarily...and it doesn't have anythin to do with anger or anything....but it's just an involuntary movement (shaking)....why is that? why does it happen? is there an explanation?