Lots of "warped" rotors are actually uneven brake pad deposits on the rotors.
This happens when braking hard for a redlight, and keeping the brakes on hard at the redlight.
Can be mitigated by using the parking brake to hold the van stopped at a redlight after a hard quick stop, as drum brakes are not susceptible to this "pulsing".
Lots of people put way more pressure on the brakes after a hard stop than is actually needed to keep the vehicle from rolling forward after stopping.
My sister, like a lot of women, are late and hard brakers, and soon after she got her rotors turned or replaced because of pulsing, they would be pulsing again.
And being a woman it simply was not her fault, as no woman in the history of anything has ever done anything wrong, ever. Modern Feminism has dictated this, and women's magazines remind them dozens of times a day.
But after my suggestion that she not keep the brake pedal clamped super hard after one of her often late braking episodes, mysteriously her pulsing pedal never returned, but she was irritated that she could not blame a Man for it and had to find other areas in which to exercise her magazine grade misandry.
Vans being heavier and loaded can really make this scenario worse than the average car.