This is one of the things that people would like to know. Some say that the possible reason that triggered this is the Michelle Fields incident. Shapiro is a Cruz supporter, so he decided to resign as an editor-at-large. Some people are also saying that Milo is considered to be a provocateur.
He tends to say things to people that will make them look foolish. It seems that he also has the power to be emotionally invincible. He is not the type of person that will quickly react to various things. Some people are speculating that perhaps they do not like each other. They have some differences that they cannot shrug off.