It goes back to that presidential roast he attended. Don't think Trump ever got over that.
it was the WHCD, White House Correspodents Dinner. Obama and Seth Meyers unloaded on Trash.
"At the time, Trump was in his heyday of promoting the “birther” conspiracy theory about President Barack Obama and hinting at a possible run for office the following year.
Taking advantage of his attendance at the dinner, Obama dug in to Trump to poke fun at the real estate mogul’s business dealings and doubts about
the president’s place of birth. Here is what he said at the time.
"Donald Trump is here tonight. Now I know that he’s taken some flak lately.
But no one is happier—no one is prouder—to put this birth certificate matter to rest than The Donald.
And that’s because he can finally get back to focusing on the issues that matter:
Like, did we fake the moon landing?
What really happened in Roswell?
And where are Biggie and Tupac?
All kidding aside, obviously we all know about your credentials and breadth of experience.
For example … no seriously, just recently, in an episode of Celebrity Apprentice, at the steakhouse,
the men’s cooking team did not impress the judges from Omaha Steaks.
And there was a lot of blame to go around, but you, Mr. Trump, recognized that the real problem was a lack of leadership,
and so ultimately you didn’t blame Lil Jon or Meat Loaf, you fired Gary Busey.
And these are the kinds of decisions that would keep me up at night.
Well handled, sir. Well handled.
Say what you will about Mr. Trump, he certainly would bring some change to the White House."
The skewering of Trump did not end with Obama —
Seth Meyers, host of the dinner that year,
took aim at the Trump for several minutes,
complete with an imitation of the future president."
https://time.com/4756751/donald-trump-white-house-correspondents-dinner/