Warped rotors. It's been discussed here many, many times.

Of course, one reason is because the rear drum brakes do not self-adjust very well, forcing the front brakes to do all the work of stopping the vehicle. This generates excess friction heat and can warp the rotors.

Another reason is over tightening the lug nuts.

But my friend just sent me this video link, showing a reason for brake shake that I don't recall ever being mentioned here before. It's close to a 30 minute video, but good information.

Short version - clean the corrosion and junk off the hubs!

 

If you don't understand runout (and TIR,) you can't do it right.

 

I musta been doing it right since 2009, no brake shake after upgrading to 2009 SS non Brembo rotors !