If I was asked to provide the "real" reasons for the 1975 phenomenon among JWs I'd give the following factors.
Fred Franz increased in confidence as well as disconnect from reality following his successful translation of the NWT. He increasinly developed detailed interpretations of prophecy as the long books on prophecy from the period show.
Knorr may have stopped the 1975 fiasco were it not for the fact that his position and authority had been weakened by the findings of the Aid book researchers in the 1960s and his failing health in the 1970s.
JW growth was pretty phenomenal in the 1940s and 1950s, but had slowed significantly in the 1960s. Even those in the leadership who were not convinced by 1975 may have been swept along by the new enthusiasm and activity the date produced.
It was an unsettled time in society in general, with the failed revolutions of 1968, the Vietnam war, economic and political instability. All these things may have made it easier for JWs to believe the "system of things" was nearing its end.
An interesting aspect of the 1975 phenomenon that is not often commented upon is the very uneven impact it had on JWs worldwide. If you look at the numbers who joined before 1975 and those who left after 1975, some countries such as the Philippnes lost up to half the membership, whereas growth in other countries such as Italy was not adversely impacted at all.