I see two reasons why I can find myself emotionally reacting to a small thing.
One is that I have several big things that I'm struggling to deal with. That's a heavy load. So when some little thing happens, it adds to it all and my reaction is not just about that small thing, but everything. For example "I just lost my job, my girlfriend wants to leave me - and I just snapped a shoelace, dammit! Aaaaaagh!"
The other reason is that small thing triggers some big feelings I've repressed. For example "That man is wearing the same cologne my drunk abusive father did and I'm feeling a lot of hate right now!"
With this second one, the reaction may be unconscious, and I may feel very strong emotions unrelated to anything going on. This can be confusing.
But in both situations the feelings are reasonable. This is why I never blame myself for how I feel. It always makes sense and is appropriate, no matter how deeply hidden this may be from my awarenwess at the time.