Why do people look to argue over insignificant things?

cause hes mean, maybe you should tell the police about it

Threatening, pushing, and cursing indicate he is not a rock-solid individual.  You might mention the interaction to your supervisor.  Being a janitor is a responsible worker.  "Shop Class as Soulcraft" by Matthew Crawford, and "Understanding Yourself" by Mark Prophet are good books.

...asked the Yahoo user arguing over insignificant things without any sense of irony or hypocrisy.

"Those who are feared are hated". That fellow must have been afraid of you, reminded you of an authority over him at some point in his life. 

it sounds like you crossed an unhinged headcase, avoid people like that & report them to the police.

For some reason he felt he had the right to confront you about it.  So sorry he vented on you, the guy needed a kicking post & you were the one he took it out on.

Because some people are insignificant sacks of ---- and they think they can hide that fact by arguing with other people.

if that guy pushed you and threatened to stab you then I wish you would have pressed charges. I am lucky, because I am big and fast enough to push back and probably take the knife away from most people. But most people might become seriously hurt or worse doing the same. The only way to get creeps like that away from other people is to have them put behind bars. If it happens again please have the person arrested.

They sound like they were looking for a fight. And making any excuse. Who knows why they are so angry and like that. It has nothing to do with you so dont blame yourself. The guy sounds like a raging lunatic who is mean to others. Super looked down upon by most people.