The WTS literature strongly hinted that 1975 was it and hyped people up. It was that hyping along with the talks that really pushed it over the edge. The talks were much more direct. As with most dub things once it is decided that spiritual people do this or that and believe this or that then it's a sign of spiritual weakness to not believe.
I think the literature started the fire of 1975 and the talks fanned the flames. Individual publishers took away the thoughts that the WTS wanted them to and pushed their activity to new heights. All while headquarters enjoyed unprecedented growth, for a time.