Fear is one of the popular causes of anger. If a person felt afraid or threatened he might become angry. If for example a reckless driver was about to crash into someone's car then the later might get angry because he got afraid.
People can become angry in response to loss of control. If a person lost control over his environment then he might become angry in an attempt to regain some of that control. For example a parent might burst in anger if the children continuously disobeyed his rules.
People can get Angry to protect their Egos and assert their authority. A person can get angry if he felt that others are not giving him the respect he deserves. Egoistic People are more likely to get angry faster than others.
Each person has a certain degree of patience and tolerance. When a person runs out of patience and becomes unable to tolerate something he might burst into anger. People who are less tolerant and impatient are very likely to get more angry (See why some people are impatient).
A person can get angry with the goal of hurting others. A person who criticizes others badly right after bursting in anger could be displaying aggressive anger where his goal is to make those people he criticized feel bad.
Frustration can lead to anger. If a person kept waiting in a queue for too long her might get angry. At this point the anger might not be directed towards a specific person because its an anger caused by the situation itself.
When a person feels threatened or when he believes someone is threatening one of his family members he will usually get angry. In such a case the purpose of the anger is to let that person defend himself or the loved ones.
A person can get angry because someone attacked his important beliefs and values. Members of a certain religion usually get angry when someone mentions their religion or religious figures in a bad way.
When a person gets very stressed he is very likely to get angry fast because his tolerance is greatly reduced. If a person is living a very stressful life then his chance of getting angry might become much higher.
Narcissistic people usually get angry much faster than others. Because those people assume an exaggerated importance they become mad at any person who doesn't give them the level of respect they believe they deserve.
Some people already live with a certain amount of suppressed anger. Unmet goals, severe disappointment and an unhappy life are all factors that can make a person always angry at life. Those people usually lose their temper much faster than others.
If a person felt very angry then he is very likely to lose his temper again in the next few hours in case he didn't calm down completely. Anger usually leads to more anger and reduced tolerance until the person calms down completely.
When a person feels that his rights got violated or that he was treated unfairly then he might become angry and lose his temper (see how to become assertive).
When a person feels rejected or when he believes that people are ignoring him he might respond back with anger. A famous example is when a person deletes someone from Facebook because that person didn't give him proper attention.
When something prevents a person from reaching one of his important goals he might respond back with anger. The frustration caused by the delay of the important goals usually leads to anger.