Because they are reacting as their 6 year old self
They lack a lot of self-awareness and live in denial. Whatever happened, that seems big to them and small to you, is big to them because it has created an emotional spark from something bigger to them that they haven't dealt with yet. When someone hasn't dealt with past issues yet, they will continue you badly deal with anything seemingly relating because, simply put, they don't even realize they are doing this; they are ignorant to their ownself.
Because sometimes it's more of an accumulation of small things that build up...but sweating the little things is a waste of energy in my opinion.