According to Roger Stone in the PBS Frontline documentary that aired leading up to the election, it was Obama's persistent jokes at Trump's expense during the 2011 White House Correspondence dinner that really amped up the animus. Trump essentially vowed revenge from that point onward. https://www.pbs.org/wgbh/frontline/article/watch-inside-the-night-president-obama-took-on-donald-trump/