If you generate a world with world type customized and set the dungeon count to a high number there is a chance that the dungeons will overlap.
When this happens there is a chance the chest(s) get destroyed dropping its loot.

How to reproduce:

Generate a world with world type customized and set the dungeon count to a high number. Look for two dungeons close to each other. There is a chance you will find a bunch of items on the floor.