Sounds like your amygdala switches your brain into safe mode, and powers down non-essential functions. I think that's pretty standard, even if the expression of it is a little peculiar("Spock mode"?); speaking directly and clearly is an effort to avoid further miscommunication.
The only time in recent history when I started shouting at someone, it was a deliberate act, because I had an employee flipping out, and speaking to him calmly wasn't working. He was acting like he wanted to get into a fight; so, I could let him hurt someone, or I could call the police(and let them possibly hurt him), or I could mirror his behavior, and see if he really wanted to get into a fight, or whether his tantrum was just an effort to draw attention to his grievance(his shift supervisor kept changing his schedule without consulting him or something like that). After quickly assessing that I wasn't getting through to him, I started making a bigger scene than he was making, and he seemed shocked back to his senses, and backed down. He was just an overgrown kid, and wanted to know he was being taken seriously, but he had everyone scared half to death with his rampaging bull elephant routine; someone else would've just had him hauled out in cuffs, and let him cool down in a jail cell. He got lucky, but he doesn't work for me, anymore(too much drama).