When I'm in a (somewhat stressful) call I feel a strong need to move. Like sitting still would make me actively uncomfortable. I assume walking around helps you destress a little.
because your circulation is increased and you breathe more oxygen. Many christian monastries have courtyards with a covered way surrounding it, so people could walk and talk. The element is also present in many other architectures.
It's not really a conscious decision. This doesn't explain the compulsion but in case you thought we were doing it for a specific explainable reason, we're not.