It's what fascists do.
Trump, predictably, threatened him.
Even at a stage at which he's trying to seem presidential, The Donald has a tough time shaking his inner bully. I really find it hard to believe that people don't stop and think about a potential president who not only promises to order our troops to commit war crimes and proposes the gutting of free speech by allowing him to sue people who criticize him, but also promises to abuse the presidency as an instrument of personal revenge for petty slights.
Goes along with his Narcissistic Personality Disorder and his low self-esteem.