Why? It's difficult to say. Maybe because of personal dislike or maybe because of professional reasons. Some offensive statements were just the answers to another insult previously expressed. Readers and fans are not agree and continue the chain if word wars between each other. Nevertheless, great writers are creative even in such common deal as quarrel. They say highly figurative phrases that can be used to imagine their characters better.
The history of literature knows many examples of author-on-author insults. Ernest Hemingway and William Faulkner, Truman Capote and Jack Kerouac were engaged in such stories. This infographic from AussieWriter writing service presents some of the most metaphorical among them. You can learn how not to be boring while insulting someone)
Infographic by: aussiewriter