I think the most pragmatic way to look at to get Machiavellian on it.

The people executing Trump's policy at this point are the ones that are willing to stomach Trump to get ahead. They stomach Trump up to the point where they no longer get ahead, at which point they stop stomaching Trump. Most of the time, Trump is the one who kicks them out at which point they snipe back (follow Scaramucci on Twitter) but lately, we've seen a number of people making the calculation that it's better to stop now while you're ahead.

Make no mistake: Mark Esper is not qualified to be Secretary of Defense. He's probably qualified to be VP of government relations at Raytheon, which is what he did before being Secretary of Defense. And at some point last week, Mark Esper decided that he needed to preserve his future at Raytheon et. al. from his present at Trump Inc. That's as a cabinet member in an administration up for re-election.

And, as b_b points out, Esper isn't alone. We've heard over and over about how the relationship between Trump and his own White House is adversarial, and how Trump's White House has an adversarial relationship with everyone else. They can afford to be adversarial because they're in power but as soon as that power is contingent on everyone extending them that power, the whole thing collapses.

If Trump wins cleanly he gets to be President. If Trump wins uncleanly he probably gets to be President - at this point I think enough allegations of voter suppression might just get us a do-over. If Trump loses, even uncleanly, this shit's over.