In my rather extensive experience there are two major causes of pulsating brakes. I maintain a number of vehicles for a fixed user base and knowing their habits and driving conditions has helped identify patterns, particularly with respect to brake issues.
The first and most common cause is "cheap rotors" which is hard to define, but usually the least expensive rotor at the store. They just seem to develop rotor warps, although sometimes they may take a couple of years to get there. These less expensive rotors tend to also have portions of the rotor metal unevenly break away, although not in a pattern that might support pulsating rotors.
The second most common cause is from users who may use their brakes enough to get them warmer than normal, and then sit stopped with substantial braking pressure for a period of time, like at a traffic light.
Users who fall into the second category have been successful in reducing their warping by lightening up on the brake pedal after stopping after long hill descents.
Solid rotors seem to have less of a problem with this than vented rotors.