Anger is usually directed at another, but always is a result of something "we did", yet much of the time there is no obvious explanation for it.
"I didn't cause that or I didn't create/choose that" is a normal response for us all. Then we come to realize, wow, our soul did choose that. One even gets angry at their own soul for choosing those things to "suffer" sometimes. Sometimes one creates new responses and realizations upon the realization that they chose that, creating a whole new process to clear.
Even after all of that work, many find that anger still resides within their own being; and for no apparent reason. The logical mind needs an answer, much of the time, unless the heart is dominant, then one just recognizes it, says thank you, and lets it dissipate.
For those who need a reason, here are two more. Anger is one that surmounts as masculine energy continues to purge and come up for release, in magnitude. On a mass level, devastation and "wronged" events, brings up anger in a huge collective purging and release. That alone serves a multitude of reasons too. (Ascending souls, every unhealed emotion surfacing as the result of anger/fear, victim mentality, pain, suffering, tears and especially gratitude, unity, love, etc.)
First, we ALWAYS have the answer within. But, in a moment of masculine energy (fear/ego) winning, we gave in. We compromised and went with either what was "easy" or more desirable at the time, over what we "knew". Then when the situation came back around, we got angry at another, or even ourselves. And then tucked it away to emerge another day.
But then, we get through the understanding that oh, we chose that, and we think "oh, smooth sailing".. then boom, anger surfaces again. "But wait, I know better than that"... the logical mind says.... so where is all this anger coming from?
It comes from the individual (lower self) realization that we have lied to ourselves by way of our own ego, logical mind, learned belief systems, thoughts.... the little self is rebelling against itself for creating itself... in an attempt to separate, cleanse and then reintegrate as a part of the whole. Our minds are angry at our minds.... go figure that one out!