A paradox: telling someone to 'calm down' always, always infuriates that person. Dropping the 'calm down' bomb is the ultimate act of passive aggression - it says "You're out of line, and I need you to act in a more appropriate manner, like me." Needless to say, this is not calming.
A: Goddammit! How could they possibly cancel Arrested Development?
B: Dude, you need to calm down.
A: Calm down?? Fuck, I am calm, what do you mean calm down? (goes apeshit, breaks lamp, exits room)
C: You HAD to drop the calm down bomb.