This helped fuel the "Paul is dead" rumors. If played backwards, you were supposed to hear the car crash where Paul died, and a voice saying "Turn me on, dead man." Also, playing the line, "I'm not in the mood for wearing clothing" in reverse eventually becomes a rather odd but clear reversal, "There were two, there are none now."
This is referencing the rumor that Paul McCartney died in a car with "Lovely Rita
" and that the two were burned away after the wreck.
The rumor took off in October 1969 when a listener called the radio station WKNR in Detroit and told the DJ Russ Gibb about the backward message. When Gibb played it backwards on his show, listeners went wild and spent the next week calling in and offering their own rumors. The story quickly spread, and McCartney helped it along by laying low and letting it play out.