Russell was not by any means the originator of the 1914 date. Rather, this was borrowed from a group known as the Second Adventists, a fact readily acknowledged by the late N.H. Knorr (as quoted in Raymond Fraqnz's Crisis of Conscience). Led by one Nelson Barbour, the Second Adventists was one of a number of groups that emerged from the wreckage of William Miller's 1844 debacle. As others here have noted, Barbour finally settled on 1914 after his prophecies about 1873 and 1874, failed to eventuate.
Incredible as it may now seem, Pyramidology was taken quite seriously during those years of the mid to late 19th Century. (Many persons, it would seem, were very taken in by an 1864 book on this subject by a Professor Smythe). Charles Russell went on to use this pseudo-science to back up his 1914 date, probably deluding himself into believing that Pyramidolgy actually "confirmed" what he had said all along about 1914.
One of the morals of this story is that if two dates line up, it means nothing at all!
Bill.