Why do people shake their legs while sitting?

it's one of many subtle coping mechanisms. if someone is aggitated by something, they'll twitch their legs. when i stopped doing it it gave me time to observe others that do it and yeah people that are more flamboyant and active will tend to do it more. or if they can't sit still. i find that if you control your breathing to breathe slower you're less inclined to be aggitated and thus inducing the leg twitching.

interesting note is: a lot of times when someone needs to use the restroom they'll do the leg twitching