For example, sometimes planets and stars, when lined up correctly, can magnify radio signals – the same way a magnifying glass strengthens light as it passes through it. So it wouldn’t be unheard of for a typically weak signal to get strengthened and look like it was abnormal by the time it reached Earth.
A second, potentially disappointing, possibility is that the signal actually comes from Earth. Crazy as it might sound, someone tinkering with a short wave radio in their basement, a plane going overhead or a solar flare could emit a signal that gets bounced around our atmosphere and redirected back to the ground. The RATAN-600 is vulnerable to these mistakes, though they are rare: a chance of about one in 5,000.