It was already expected that should Trump take a second term in office a key cabinet post for immediate turnover would be Defense Secretary, especially given Mark Esper and Trump have been known to be publicly at odds.
NBC on Thursday afternoon cited three unnamed current defense officials to say Defense Secretary Esper has prepared an undated letter of resignation, also ahead of a possible presidential transition, which is common practice at the end of an administration's term even if they stay on.
However in Esper's case he's definitely expected to be pushed out following the election. This also after protests and riots broke out on American streets through the summer. Esper reportedly strongly opposed Trump efforts to send active duty troops to hot spots that had seen widespread rioting and looting.