"My" camera repairman, who is very experienced (just retired after a lifetime of practice) told me that the problem often occures after the lens have been exposed to heat. Most typical case is when the camera and lens have been laying in the car, exposed to sunlight.
That's why many long teles have a white or silver colour, to prevent thegrease from "steaming" when the lens is standing on a tripod in the sun for hours.
my 2 cents
a lot has been said here so i'm throwing in my ¢ worth.
when you store your lens, store it with the front of the lens down. the ap is at the rear.