Rat's Nests are junk assembled by rats, which quickly scurry away when their home is torn apart.
Skullpiles are heaps of skulls most likely put there by demon after they extracted them from the human heads that had contained them. Perhaps the loot was stashed away there by whatever demon was stacking them.
Rock Piles are heaps of rocks, it is not known why these rocks are piled up, but they are a more rarely seen type of pile.
Goo Piles are large gelatinous globs of green goo. They are found in great number within the underground lairs of Sand Maggots, suggesting they are a product of the oversized bugs, however they can also be found in the Lut Gholein sewers, where the insects are not present. Goo Piles are often trapped, but the trap will always be a poison cloud.
Cocoons are piles of spun silk surrounding something, perhaps intended as food for baby Giant Spiders which scurry away when the Cocoons are violently kicked open by warriors seeking the possibly valuable contents.