Trump, who was incensed about the rising levels of migrants and threatened to shut down the southern border on Twitter earlier that morning, took Bolton's side during the argument. Bolton favors a harder line approach to the issue and criticized Department of Homeland Security Secretary Kirstjen Nielsen during the argument, a source said. Nielsen used to serve as Kelly's deputy when he ran DHS. Bolton reportedly said Nielsen needed to start doing her job, which incensed Kelly.
https://www.ktvz.com/news/politics/bolton-and-kelly-get-into-heated-shouting-match/809928138