The cause of the differences in this case is that not all manuscripts have 'the Son' in them and the KJV was using one of those. Sometimes differences in manuscripts are simply a copyist mistake based on the similarity of given Hebrew or Greek words, but that does not seem to be the case here. What we have here in my view is a typical well-intentioned yet foolish mistake in trying to protect the divinity of Christ.
I imagine the foolish copyist thought Jesus must know everything since He was God, so the words were erased. Bruce Metzger's Textual Commentary1 supports this assessment:
The omission of the words because of the doctrinal difficulty they present is more probable than their addition by assimilation to [the parallel passage in] Mk 13.32.
This omission is foolish, however, because on many occasions the human nature of Christ did not know many things, while we know that his divine nature always knows all. His human nature is not omnipresent either, nor is it immutable, for it grew and changed. In any case even removing the words still includes Jesus as not knowing the hour because no man knows the hour and Jesus was not only God but also a man.
In other words, the KJV is not correct in this instance and the majority of other translations have probably relied on more accurate manuscripts. However whenever a difference exists there will remain some debate. Luckily there are not enough differences among manuscripts to alter the understanding that we are able to obtain from them all. It is always this kind of minor stuff that can be explained better from other passages.
1. Bruce M. Metzger, A Textual Commentary on the Greek New Testament. New York: United Bible Societies, 1994, 51-52.