Looks like she got a little drunk at an off the record meeting with reporters and said too much. And of course, she made the mistake of trusting the reporters.
According to Politico:
Madeleine Westerhout, who left her White House job suddenly on Thursday as President Trump’s personal assistant, was fired after bragging to reporters that she had a better relationship with Trump than his own daughters, Ivanka and Tiffany Trump, and that the president did not like being in pictures with Tiffany because he perceived her as overweight.
Given Westerhout’s sensitive role as a confidante to the president, the few details the White House shared about her abrupt firing had Washington’s political-media class in a quiet frenzy on Thursday night and Friday.
The critical comments happened at an off-the-record dinner, according to two people familiar with the matter, that Westerhout and deputy White House press secretary Hogan Gidley held earlier this month with reporters who were covering Trump’s vacation at his golf club in Bedminster, New Jersey.
Westerhout also jokingly told the journalists that Trump couldn’t pick Tiffany out of a crowd, said one of the people. “She had a couple drinks and in an uncharacteristically unguarded moment, she opened up to the reporters,” the person said.