Shudder comes from a few different sources:
1. Pad imprinting, from improper break-in or racing followed by complete stop(s) with very hot brakes.
2. Uneven pad build-up.
In both cases, the rotor develops more friction where there is more embedded pad material, and less where there is not.
If a rotor is mounted on a hub and not dialed in for minimum runout, this will happen on the high spots. Worst of all it'll happen on alternate sides/alternate ends of the rotor.
That's why every Factory Service Manual I've seen recommends checking runout on installation. The tool to use is a dial indicator. You can get a cheap one you don't need Starrett or Mitutoyo.
Your typical dealer, local shop, or home mechanic will gleefully ignore the FSM and tell you that "it's set at the factory" "never had a problem" and "we use quality parts" and of course you'll have the problem down the road, because it takes time to build up and develop. Unless the runout is really bad! Then they'll accuse you of running through a puddle with hot brakes, and charge you for another brake job. For the auto service industry, it's the gift that keeps on giving.