The load you were already carrying

The same trigger does not produce the same response twice, and this is not a mystery.

Five hours of sleep. A back that has hurt for three days. Nothing eaten since fajr. Money. An argument from yesterday that was never finished. Any unpleasant state at all, including plain heat, makes anger arrive faster and land harder, whether or not it has anything to do with what just happened.

This is not an excuse and it does not move the responsibility anywhere. It is the most workable thing in the course, because you can change some of it deliberately, and lesson five is about which parts.