If I remember correctly, it was originally to do with parallels between Jesus' first advent and the second one. Very roughly:
29 CE/1874 CE - Jesus' baptism and ministry starts/presence starts,
33 CE/1878 CE - Jesus hailed as king riding into Jerusalem/Jesus' installation as king in the heavens; Jesus inspected Jerusalem's temple (threw out the extortioners)/Jesus inspected spiritual temple; also the "church of the Scribes and Pharisees" 1 was cast off/apostate, Babylonish Christendom was cast off. 2 Also resurrection of Jesus/first resurrection of the Christ class.
Later in Ratherflawed's time, these parallels were transposed onto the 1914-1918/19 period and built upon.
1 As at the end of his three-and-one-half year's ministry, Jesus wept over that church, gave them up and said: "Your house is left unto you desolate," so we believe that at the parallel point of time--the spring of 1878, the nominal gospel church was given up and their house left desolate. Until the harvest of their age, all outward Jews--"wheat and chaff" were spoken to and of as Israel but Jesus did a separating work--"His fan is in his hand" to winnow the chaff from among the wheat and when the work of separation is accomplished the wheat only is recognized. So here true and false grow side by side in the church and together they are nominally "The kingdom of Heaven"--"Let both grow together until the harvest"-- but in the harvest of this age "He will gather out of his kingdom all things that offend," &c. 'Matt. 13:30,41'. But since the spring of 1878 we believe that the nominal church is cast off and now only the wheat--"the little flock"--is to be considered the church and we believe this to be the date of the beginning of the fall of Babylon church, the parallel to the date of the fall of the church of the Scribes and Pharisees--hypocrites, who encompassed sea and land to make one proselyte." Their fall was gradual, so is Babylon's. - ZWT 1879, Nov., p. 2 [R46].
2 'To our understanding, as already shown in the MILLENNIAL DAWN series, 1878 marked the date for our Lord's assumption of Kingly authority and his judgment upon Babylon the Great, characterizing her as "fallen," and calling for all the people of God to come out of her: and that it marked also the date for the faithful overcomers of the past to have a share in the first resurrection--to enter into the joys of their Lord, and hear his words, "Well done, good and faithful servants."' - ZWT 1901, Feb. 1, p. 60 [R2765].