Looking at them, they all appear to have been detected by SDSS (Sloan Digital Sky Survey) so I would assume that it is just a bias towards the Sloan survey looking at that part of sky?
Wow!
Rather reminds me of a similar graphic illustrating each nuclear weapon test around the globe from 1945 onward: https://www.youtube.com/watch?v=LLCF7vPanrY
Of course, the energies involved are rather less than with SN...!
Dean