Simplistically speaking, things that bring out anger are referred to as anger "triggers". They are similar to stressors but specifically make you angry rather than worried or nervous.  In Middle School, you may have done a small group assignment listing your anger triggers.  It is important to know what "gets under your skin", "pis**s you off" or whatever phrase you want to use so you can better prep your control.      

When anger is brought out by external events, it may be a specific act done to you or a specific act you witness.  The action that makes you angry may be something that directly affects you or that affects another loved one.  It may also be a known injustice in society.   In some cases, Anger is valid and justified.  Other times, it's not.  

Anger may also be a lot of little things that people do that you generally find annoying but when added up, make you feel angry.  This is often referred to as "pushing your buttons".  A quote I like to say is "even a drip can make a pitcher overflow". Why?  Some people hold in their anger and instead of it going away..it slowly builds up.  A lot of little things can eventually lead to a big blow up.  Not cool.  

Other times, Anger is an internal issue.  It may stem from other emotions such as frustration, anxiety/stress, hate, jealousy, or sometimes guilt.  It may also manifest due to other issues with one's self including a poor self-concept.  In addition, hormone surges may also make people feel angry quicker for no valid reason.     

Let's think of this example for a minute...You know, the actual lightbulbs in stoplights aren't actually red, yellow and green right?  You know it's white covered with a lens over it that makes the stoplight look like different colors.  One of the phrases people sometimes say when they are mad is they are "seeing red".  That's kind of how anger works a lot of the time. A lot of us have an "angry lens" that makes our emotions look like anger on the outside. But if you take that lens away, the real emotion might be a totally different hue.  A lot of the time, our anger might actually be the outward behavior "triggered" by other emotions.  It ends up being our body's defense mechanism because we aren't really dealing with the emotion in question.    Maybe we're actually feeling scared. Maybe we're feeling guilty. Maybe we're ashamed. These are really personal, vulnerable things to be feeling and we don't have a lot of practice in sharing them with other people. So, instead, we put on our "angry lens" and fool everyone around us into thinking we're just plain mad. Sometimes, we can even fool ourselves.  but other times, it's actually things that get us angry.  Sometimes this anger is justified and sometimes, it isn't. 

Wait..the second paragraph said sometimes "anger is valid and justified"?   YES!  ABSOLUTELY!  Like any emotion, being angry or getting angry is not the issue.   Some articles actually suggest that anger is an essential and necessary emotion and many articles discuss "righteous anger" such as when someone is angry because they witness bullying or racism.    It's not anger that is important, it's how you express or deal with the anger.   Again...it's not whether or not you get angry...it's what you do when you're angry that is the issue.  

The BIGGER QUESTIONS to ask yourself:             

Do I have a right to get angry right now (can also be worded as - is my perception of the situation making me angry accurate and valid or am I reading into things)?

If the answer is yes then... 

Do I have a right to act the way I’m acting because I’m angry right now?

How will my actions harm someone else?

What may be the consequences of my actions if I react rather than respond?