I have spent a number of years as a forensic metallurgical engineer in my life. The number of reasons that steel corrosion occurs are endless. Each individual case of excessive corrosion must be evaluated on its own history. One should not try to make a general observation that encompasses all steel corrosion. The corrosion could be caused by stray electrical currents, variations in steel composition, as Steven suggested, the composition of adjacent materials and even to the air that the steel is exposed.