When Obama became the Democratic nominee and then president, Schlump couldn't stand it. He could see that this guy was smarter than him, better looking than him, more athletic, more charming and witty, 15 years younger than him ... AND black! Insecure people don't like people who make them aware of their own shortcomings, and Schlump is one of the most insecure people to ever walk the earth. He would never, ever admit if to anyone, even himself, but on some level he has always known that Obama (a black man!) is far and away superior to him in every way, and he. Can. Not. Stand. It.
When Obama made fun of him at the White House Correspondents' Association Dinner, I think that was the final straw. He hates being the butt of a joke more than just about anything and he absolutely could not tolerate that coming from a man he was already desperately envious of, especially a black man. I think that was a major narcissistic injury for him, and he has never gotten over it and never will. He's wanted to get back at Obama ever since, but all of his attempts have rolled right off Obana's back. If any of his insults has bothered Obama in the least, he's never let it show, and that has resulted in a long smoldering rage that Schlump can't do a thing about, other than make fun of his name, and he never misses a chance to do exactly that.
If I sound like I'm harping on the race issue, that's because I think it's a major factor here. Trump is such a racist that being shown up (as he sees it) by a black man is a YUGE slap in the face to him, much worse than it would be coming from another white man. It's pretty pathetic, but Schlump IS pathetic.