It's not so much heat as it is rapid and marked change in temperature. Ever hear an ice cube crack when you put it in a glass of water? That's because the instantaneous temperature differential causes the ice to warp and crack.

Imagine slamming really hard on the brakes, or using them on the highway slowing down from 70mph to 30mph (exit ramp), and then running through a puddle of cool water. The result is going to be warpage. There are a hundred normal everyday-type driving scenarios under which the same result can occur. Which is why I go with the cheapest rotors I can find.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280