According to CNN,
Ivanka Trump, the candidate’s daughter, played a pivotal role in convincing Trump to take action against Lewandowski. She had been critical of Lewandowski for months and this was not the first time that she had leaned on her father to part ways with his top aide. Contributing to Ivanka Trump’s recent dissatisfaction with Lewandowski were intensifying tensions between Lewandowski and Ivanka Trump’s husband, Jared Kushner, an influential force behind the scenes. One source said rumors swirled that Lewandowski had attempted to plant negative stories in the press about Kushner — a final straw for Ivanka Trump.
By this point, Eric Trump, Ivanka Trump’s brother, had already reached a breaking point with Lewandowski and expressed his frustrations about the campaign operations to his father, according to a senior campaign source.
It was ultimately Ivanka Trump who sat down with her father on Sunday and convinced him to let Lewandowski go — even offering an ultimatum of sorts about her own continued involvement with the campaign, according to sources with knowledge of this conversation.