Sudden ABS Rear Brake Application

[waybomb]

Hi all

1987 420SEL.

All of a sudden, the rear brakes applied themselves. No ABS light lit, and only the rears applied. Shut car off, restarted, problem has not reoccured.

But now, every once in a while, the ABS faintly actuates on a slow stop.

What would cause the rears to lock up? How to diagnose? I am guessing the fronts are simply dirty senors or sendor wheel.