I've heard from Bethel insiders that what actually happened is that Freddie Franz was surprised when his book Life Everlasting in Freedom of the Sons of God generated so much enthusiasm about 1975. After all, earlier books and Watchtower statements had said almost the same thing, but not with quite as much gusto. Then Franz himself got caught up in the enthusiasm and so did many other Watchtower leaders. They saw an opportunity to build enthusiasm for their religion, and used it to the full. It worked, because the Society had never seen such a big increase in such a short time.
For a fairly comprehensive look at what the Society said about 1975, take a look at the essays "Notes on False Prophets" and "The WTS and the End of the World" at http://www.geocities.com/osarsif/index2.htm .
AlanF