tl;dr the signal appears to have been from a cold hydrogen cloud "resonating" off of radiation bursts; namely, those emitted by neutron stars. The stronger the burst through the cloud, the louder the signal on equipment. The WOW! signal appears to have been the result of a particularly powerful event, but by observing the same/similar (?) gas cloud(s), they've been able to spot signals with the same signature, albeit weaker due to being hit by less rare (and less powerful) phenomena.
Some clarification might be needed on whether it's a specific cloud that produces this signal, or if any cold hydrogen clouds are capable of it. I couldn't seem to find any in the article itself. Maybe there's something in the published research paper that provides further information.