When playing in a HAM compatible RoG world I noticed that a puff of smoke(Hamlet cloud border effect) at the sea border. All the pre-generated items on the ground slowly made their way to one side of the map for some unexplained reason. I've never experienced this before in RoG, so I figured it was a bug.
edit: both of the things above happen in the caves too.
the puff of smoke comes from items crossing into the sea/void
edit 2: Earthquakes in caves don't shot items falling, just shadows and the occasional mole.
Start a RoG world and set it to HAM compatible. I had extra boons, touchstones, extra bees, treeguards and poison birchnuts. (custom world in other words)