This seems to have been long-prepared. The new acting head is expected to be Mark Morgan, the acting head of Customs and Border Protection, according to Molly O’Toole and Molly Hennessy-Fiske’s report in the Los Angeles Times. They write:
Morgan worked his way back into Trump’s graces in large part due to Fox News appearances in which he praised the president and took a harder line on immigration and border security that surprised some former and current colleagues.
It’s hard to be toadier-than-thou in the Trump administration, but Morgan seems to have pulled it off.