Rotors don't actually warp. Well, there may be horribly low-quality ones made of a dangerously cheap metal that can warp from the sudden and drastic temperature fluctuations. But, the temperatures required to soften and melt the metal to cast rotors is far higher than what they experience when braking. What really happens is brake pad material burns itself to the rotor's surface, creating a high spot that is only measurable with a precision micrometer. Over time, this high spot(s) collects more pad material and grows. This makes the rotor extra thick in that area/those areas so they grab and release the pads as they pass through the caliper. This causes the pulsating feeling in the brake pedal. Cheap rotor material does play a factor as it can be a lower-quality iron that is more "adhesive" to the pad material. The pad material is what's primarily at fault, though. Sometimes, it's not necessarily a low-quality pad material as it is the pad material not being a good combination with the quality of the rotor's iron.
Improper installation can cause the same thing. If the rotor isn't seated perfectly flat against the hub due to dirt, rust, etc., this will make it wobble. It may be so minor that it's undetectable at first and the brakes may feel perfectly smooth. But the misaligned rotor will drag the pads more on that side that isn't flush and kick start the buildup of extra pad material on the rotor that gradually increases over time until it is able to be felt.
It all boils down to using quality parts and having them properly installed.
