Hundreds of reasons. They dont really love you, they have something going on and lost all empathy, they have their own issues, they have never seen a healthier relationship (right from childhoof) and think its normal, they are bastards, they want to break you down, they enjoy you being hurt.....the list goes on and on.
When I first met DP we had a brief fling during a disagreement he threw some names around. I wasnt having it. Over the next few years, he went for counselling.
He grew up in a house where name calling was entirely normal. As was hitting eachother. His mum left and the his dad got remarried and his dad and step mum were the same. Dp then got married which was an unhappy marriage where both of them did the same. It wasnt until he had extensive counselling that he realised it wasnt normal and he hated it. We stayed friends and we did get back together. He is still in counselling and has never done it again . It was like he genuinely didnt know you can disagree or even argue without both parties being as horrible to eachother as possible and seeing who can throw the worst insult.
I think when I didnt respond, during the first disagreement, in kind and ended it, it shocked him. His sister agreed with me and told him he needed to sort himself out. She had counselling previously which had helped her. He even says now he didnt realise romantic relationships could be so calm.
It's very sad. However, whatever the reasons are, no matter how understandable they are....you dint have to out up with it. Dp knows, if it happens again he is gone. I wont be insulted by the person who is always meant have my back. If he dislikes something I do, we talk about it. No name calling, no deliberately trying to hurt each other. And we do the same when he pisses me off. 