Why does a spirited honest difference-of-opinion discussion sometimes turn into insulting attack? What triggers it?

You wouldn't know spirited, honest debate if it bit you on the butt. Your version of "debate" is one reason why there is so much strife and violence in the world. Vicious, demeaning name calling is not, nor has it ever been, debate.

Lack of intelligence.

They're losing the argument so they go ad hominem. .