Why do engineers shake their legs behind a desk??

It's a habit some people have. When I'm really trying to get a lot of programming down I am trying to shut out distractions so I can focus on what I'm doing. For me this means having everything just so, my monitors a certain way, programs laid on it a certain way, headphones on, music loud, some sort of snack and drink right next to me. Anything that seems off bothers me and I'm sure I do some weird things physically while I'm programming, likely move around to the music I'm listening to, but I don't really think about it. Thats just what I do when I'm trying to shut everything out. People who shake their leg likely aren't thinking about it, it just happens.

Engineers usually drink a lot of coffee / tea, thus are hyperactive lots of the time. A lot of them are nervous, have a tick, find it relaxing. Sitting in the same position for hours on end also does not help.

Funny - I haven't smoked in months and I've noticed myself doing more leg shaking now.

I think it's a combination of factors, one of which might be that the lymphatic system requires motion to cause it to flow (it has no "pump"). So "fidgeting" feels good.