1. The way you treat me. If you treat me like shit... I will not hesitate to do it back.
2. One word. Stupidity. If you are a fucking retard, and I am trying to talk to you... if you hear me yelling: Shut up.
I usually stay calm, but I get really angry when someone talks to me in a disrespectful manner. Also, I hate scammers a lot because they are dishonest and thoughtless. It's tough to handle when people take advantage of others just to benefit themselves..
It varies from one person to another but for me it always happens in cases where the other person lies to me, constantly ignores whatever I have said or disrespects me without any valid reason. Being irresponsible about one's deeds is another thing which annoys me. However, I always try to restrain myself from reacting straightaway because sometimes it works better.