Ar, ar, ar!
Is somebody out there saying "engage"? lol.
The most likely natural explanation is some sort of beaming, along a narrow axis, that happens to be pointed in our direction, from AGN or BH events.
Thing is, without beaming the intensity in each event needs to be very high (though AGNs may handle that). But the frequency is the observed one. If there is beaming then the energy per event could be much much lower (perhaps vanilla BH level). But there would also be far more of these events going on than we see, because there would be other cases pointed in other directions.
Thing is, if that were the cause you'd expect them to become more common as the universe got older (more time to form galaxys and compactify big GN BHs), not less. But the ones we see are all pretty darn far away.