Age and "cure" of the emulsion, ambient humidity during storage and even in-camera, whether or not someone has a voodoo doll of you and sticks pins into it. I wouldn't necessarily blame any one film type. Curling of roll film is more an issue for me when spooling them onto the developing reel; and that's when I might notice a difference between a long desert trip or here on the foggy coast (I'm wearing a coat today, on a June afternoon). Very low humidity tends to stiffen the film and cause more curl.