I think it's caused by emotion, fear, and sometimes ego.
People don't like to be wrong.
People read the comments of others when a question is asked and become angered or disgusted by another person's words.
I use the word "ego" here cautiously because sometimes someone may have formed an opinion due to life experiences. So it may not be ego-driven, it just may be a valid oppositional stance.
If two people have different arguments to a side, based on their own opinion, sometimes people are left feeling wounded. They don't stop to think about trying to control their own reaction.
Sometimes it's best to say nothing and move on. No one has to prove a point here in a discussion. It's only HubPages.