A good way of relieving it (random tip from a smear nurse!!!) was that if you relax your jaw and let it drop then it relaxes the rest of you.
I think all kinds of things can contribute especially if you're busy or have a lot on your mind.
I often find I'm tense after a mad day at work and when I realise I make an effort to relax, some controlled breathing, the jaw thing, some stretching and moving my neck.
I think many people can pick up on body language though they may not be able to interpret the reason for it.
If I've had a mad day and am meeting people I'll often go for a coffee and chill out for a bit and relax before meeting them so my work stress (love my job but have so many things on my head ends up full of stuff) isn't being brought into it.