What is the cause of earlier rear wheel lock-ups in the wet? I have faith in Sheldon and Jobst's arguments about hydroplaning.

I do not understand the action of a wheel locking up (entering a skid), other than the intuitive level of braking ~> locks up / doesn't lock up.

I do not understand why the rear locks up more readily on wet asphalt.

Today my rear wheel locked up much more readily, and surprisingly sooner than I would have expected—given that I expected early locking up behaviour.

Why does the rear wheel more readily lock-up on wet asphalt than on dry asphalt.

I've already read this answer: What is the recommended type of tire for riding on wet pavement?

I'm running 47-559 marathon pluses, the bike &amp; load is 20kg ish, I'm 110kg ish.

The ideal answer would give both the science, and render it in a form that a humanities cyclist like myself can comprehend even if by analogy.
When pavement is wet (or icy, or loose, etc) it has a lower coefficient of friction.  Which means that less force is necessary to break from static state (tire gripping the pavement) to a sliding state (tire locked up and sliding).  Braking forces rely on static friction, or the grip the tire has on the pavement.  It may seem contrary, but while you are riding your tire surface enjoys a static relationship with the ground.  The surface of your tire rolls, but each individual patch of tire touches the ground and remains in contact with it (your contact patch) as the tire rotates around.  When you brake with a lower coefficient of static friction and generate enough force, you overcome that static frictional force and move to a sliding frictional force (your tire slips).