One of ther common things about things like bags of holding and such is that many of them, if nested together, will explode and rip everything into a plane-next-door; usually this is the Astral Plane.
The Plane of Filth is the result of millenia of these accidents across the multiversal landscape Gradually, the trash has drifted together and congealed into a demiplane or protoplane, adrift in the Astral mists. Due to the large number of magical items lost through these accidents, the plane itself has a potent magical field - which has, in turn, imbued it with a limited form of sentience, enough that it roams the Astral, actively seeking magic that it can absorb to make itself larger and smarter. Eventually, it stands a chance of becoming a full-fledged plane, one with the intelligence of a deity - an oddity, to be sure, but stranger things do exist.
Imagine losing that one Really Vital Relic, and discovering you have to search trough this foul, dripping, gooey plane to find it - and the plane itself can marshal 'children' to defend it, like golems shaped from the muck and ooze...