Perhaps what you perceived as someone being emotional was actually someone who spent a long time barely functioning due to things that have been happening in their life, and that small event just so happened to be a straw that broke the camel's back.
People around you are going through stuff. Everyone is. Someone's stuff is easy to carry, someone else's stuff isn't. "Back in the day" people also carried their burden with them. This is nothing new. Perhaps you never noticed because so far everyone you dealt with was great at masking, and desperately want to avoid having to see coworkers like you worsen their burden by doing what feels like kicking them while they are down.
Of course we should be compassionate, but anyone needs to be able to function enough so that they can internalise criticism and evolve.