Hi Chaps,
How about this for a "theory".
The 'Birds brake system is fitted with a PCV (Proportional Control Valve). The object of this valve is to sense deceleration forces and limit the amount of pressure being fed to the rear calliper. The way I see it, is that if you apply the front brake gently at first, the PCV will sense the deceleration and start to limit the flow of fluid to the rear calliper. If you the increase the braking force, very little if any of this will then be relayed to the rear calliper because the PCV will of interrupted the circuit, so no rear lock up should occur, unless the rear wheel is completely unloaded.
Now, if you suddenly pull a great big 4 fingered handful of front brake, some of this pressure will be fed to the rear calliper before the PCV has time to sense deceleration and act upon this, so the rear calliper will already be braking hard, then as all the weight transfers forward the rear wheel can't transfer enough of the force through to the ground and will lock up. The PCV can only stop the flow of fluid/pressure, it cannot relieve pressure once it has been applied.
I feel that if you do have a rear lock up it is either because of the above scenario, or because your PCV is faulty/sticking.
If you are getting rear lock ups on a regular basis, try altering your initial braking technique, and see if it helps.
*I reserve the right to be wrong.
Stuey