Shaking leg? Only when I'm there?

The guy I like is kind of bouncy anyway but often when I sit down near him his leg starts... almost vibrating lol and I'm not sure he realizes he's doing it. It especially happens when he's sitting close.
Most of the time he sits right up against me and generally is in my personal space.
Guys why do you normally do that?
Shaking leg? Only when I'm there?
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