Just age, and accumulated miles on the trucks cooling system. Nothing lasts forever. Radiators become far less efficient when a truck has over 200k miles causing cooling system issues. I don't think the fluid is as big an issue as it's made out to be. Sooner, or later water pumps, and thermostats need replacing, the old coolant is replaced anyways with new when thats done. Radiators are forgotten about. Engines eventually just wear out depending how they're used. Trucks are bought to do heavier work than cars, so expect them to have major issues sooner. I guess if one buys a truck to be used like a car, it'd be different with the drive train lasting much longer.