Anger is a secondary emotion, in most cases its preceded by another emotion, even for a brief time.
Hurt, resentment, pain, disappointment, etc.
I can say that, usually when we get angry it's when we're treated unjustly repeatedly. Our ego can't stand it. The little voice wants to scream.
If you mean biologically I couldn't say.
have a conniption over not getting what others are saying to me