It sort of is for me. If something someone does or says triggers anger in me, I'll shut down to them completely without caring how weird or rude that seems towards them. Though with me it is also just the way I am.
I think it's pretty natural for anger to follow shyness. Shy people are sometimes "dominated" and this can definitely cause anger (though it doesn't mean the shy person was ever actually less than the "dominant one")
I saw what I wrote above, which is true, but it is also true that my level of "silence vs. talking-a-lot" varies with my mood. Perhaps it even varies with my mood swing, me being bipolar. There is another mood in which, if I become nervous about anything at all, instead of clamming up (which would be one response) I begin to babble almost incessantly.
I'm not sure which is worse. If I clam up, I can suppress feelings that might be better expressed before they magnify and get blown up out of proportion in my mind. But if I immediately start "babbling," it annoys and confuses people around me, and leaves me feeling embarrassed -- and more likely to "clam up" in the future.
The "clamming up" will often lead to some kind of later explosion; so yes, I think it may well be a "form of anger" (or at least a means of *dealing* with anger.)