What is the cause of earlier rear wheel lock-ups in the wet?
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).