There are so many reasons lying behind anger. But the root reason of anger is Avidya, Ignorance, delusion, viewing things/phenomena as they appear but not as they are. We forget that every thing/one is transient, impermanent and like passing cloud. Forgetfulness of this truth causes anger.  Anger is a symptom of deluded mind. Anger arises when we perceive any thing/one wrongly. Anger can be overcome by Right understanding, Love and Compassion.
When a thing doesn’t happen according to a person, one gets angry. When one loses control over a thing/person, in this case also, one gets angry. Such person wants everything should happen according to him/her, in either case anger arises. But the nature seems to be vulnerable in outer sense, and causes anger to deluded mind, though it’s most systematic.
We get angry over kids, because they don’t behave according to adults, we don’t give freedom to them to grow and manifest as their soul want, we want to impose our understanding, religion, belief systems, and norms over them, and if they don’t appear according to us, we get angry.
We get angry over our juniors, servants if they don’t perform as per our expectations, and become violent to them, which ruin their intelligence as well as ours.
An intelligent one doesn’t get angry though things don’t happen according to his expectations or plans, and consequently become violent, rather he waits and believes in nature, which allows the things to happen that much only which are beneficial for him as well as others.
Reason for anger is just delusion, dullness, non-belief in nature / truth, believing he/she is controller of all things, a really thick and solid ego, to believe that things/ones are dependent on him only. Possessiveness is also a cause of anger. If one can allow the things/ones to develop, to manifest freely, then anger won’t take place.
Believing things/persons as permanent is also a cause of anger. We don’t want anything to decay, to desert ourselves. We want the things should remain forever with us, the joy should remain always with us; we forget that everything/one is transient, and can’t be made permanent, and obliviousness of this truth causes anger.
On periphery, there seem to be multiple reasons of anger, but in the root there is only single reason which is DELUSION.