Why does gravel remain static in the Nether?
This phenomenon can be seen with both gravel and sand in any dimension and occurs when the block is spawned by the terrain generator in a place where it would be expected to fall down but does not recognize that it should fall until it receives a block update. The first block to start falling will then trigger block updates for the blocks it touches causing them to fall and trigger block updates for more blocks, until all the blocks that should fall have.