The answer essentially boils down to an inability to regulate and poor boundaries. When something stressful happens, two things have to take place in order to prevent us from losing our cool.
First, we have to be able to stay calm and present, and tolerate whatever emotions we may experience in the stressful moment. Second, we must be able to feel safe and appropriately distanced from the source of stress. If either one of those is missing, we are likely to get angry quickly.
For example, if you are driving in a car and someone cuts you off, you may momentarily feel stress. If you are able to notice the emotions that come while still staying calm and cool on the road, you will probably not be angry.
You may not like that you got cut off, but you may assume the person made a mistake or just not be bothered much. If you also knew that, even though you were cut off, that you were making the adjustments necessary to keep you safe, then again you will probably be regulated.
However, if you took the cut-off as a personal attack, you may lash out at the other driver. If the other drivers actions made you genuinely feel afraid, you may also react angrily as a instinctive reaction.
If when you were cut off you feared for your children in the car, and then briefly thought about how you would be impacted if they died, and then got terrified that your children may die, then it is highly unlikely that you will be able to stay regulated and, thus, will react intensely.
Everyone is allowed to get angry. Those that get angry quickly tend to also have difficulty feeling