I have known of this one myself for years, and have heard all sorts of theories for it.
Because of the timing some suggest it was one of the Voyager probes, but this is impossible as this took place just before either one was launched. But possibly it was a satellite or moving space probe of some kind, and that is why the signal has never been picked up again (it moved on). Now, whether that probe was on Earthly or extra-terrestrial origin is another thing altogether. If Earthly, they should've figured out what satellite/probe was the culprit long before now. An ET probe could've come from anywhere and gone to anywhere, like the spacecraft in Rendezvous with Rama. This seems unlikely but not impossible.
It could also have been a purely earth-origin radio signal that bounced off the ionosphere and into the reflector dish (probably so garbled it was unintelligible) in a million-to-one fluke; such signal reflections from long distances HAVE been observed before by people with ordinary radio sets; this is basically how ham radios used to operate. Though not very exciting, a mundane explanation similar to this IS imo the most likely one.