Warping of the disc is very rare. It's one of those things that are just given as a reason for giving you new discs.
A more common reasons is for a build up of coating and dust from your pads.
Taken from an article about brake systems:
"if a pad is not properly broken in (yes, this is a thing), the material that transfers between the pad and the disc can do so in a seemingly random, uneven fashion creating islands of deposits that keep growing, leaving high and low spots on the disc. Another problem is if you hold the pad against the rotor after intense braking or coming down from a high speed, the pad can literally leave a print of material on the disc like the image from StopTech above shows.
The other way your rotors can feel warped happens when your discs develop heat spots. Modern cast iron rotors are an alloy of iron and silicon mixed with particles of carbon. At high temperatures, spots of silicon carbides form and create uneven hot spots, growing in temperature faster than the iron around it. Once this temperature reaches up around 1300 degrees Fahrenheit, the cast iron around that area begins to form cementite, or iron carbide which is very dense, abrasive, susceptible to cracks, and conducts much more heat than the cast iron around it. Once the cementite forms, continued use will just heat up those spots, causing them to heat the iron around them and form even more cementite. It's a vicious cycle."