The entire sector is completely destroyed. It is unknown what exactly caused it, but the destruction was obviously deliberate and done by an alien species, for whom a name Destroyers
was coined. Their true nature is unknown and up to debate.
The destruction that the Graveyard suffered was thorough. All stars were detonated into supernovas. Many subsequently collapsed into an unnaturally large number of black holes that can be throughout the sector. What survived from the planets, asteroids and moons that used to surround the darkened stars, was then thoroughly fragmented by a sustained MAW bombardment.
It is unknown why Destroyers obliterated an entire star sector - and probably a lot of space surrounding it.