1975 represented 6 "days" of 1000 years since creation and the seventh "day" , symbolically the "day" of rest, was going to be the Millennium. So 1975 was going to be the year of Armageddon.
There was a lot of it going about at the time. The seventies was a time of upheaval. A lot of "Last Days" books were published by mainstream Christian authors, including: "Armageddon, Oil and the Middle East Crisis" (1974) by John F. Walvoord, "The Late, Great Planet Earth" (1971) by Hal Lindsay, and "What in the World will happen Next" (1974) by Salem Kirban are some examples. The JWs were just one group among many who were trying to tie interesting times to Bible prophecy.