I’m nervous to answer this.
Here, as well as in the outside world, I think it can happen when one party participating in the discussion takes something too literally, or considers it a direct attack on themselves as a person (takes it personally), or when it strongly compromises their beliefs and/or morals (and they feel as if they need to defend as opposed to discuss them). There’s a difference between fighting and discussing. I think it’s possible to discuss something and not let it get to the point of fighting. No one says you have to agree, but you should be polite. Just keep in mind that if you debate respectfully, others are likely to do the same.