If blocks are broken while there is a block above them, there is a much higher chance in recent version/snapshots for the items to spawn inside the block above. As such, the block goes through an "Item Elevator" type situation in that it migrates all the way to the top of the column of solid blocks.

Sometimes, just from walking around at the surface of the world, I can find huge numbers of blocks that were dug out from caves below. This results in increased entities that aren't picked up, loss of resources for the player and unpredictable game mechanics. The in-game solution I have found is to always mine the blocks above the resources I find before mining the resources themselves. It's tedious.

This doesn't happen 100% of the time, but in previous versions, I had never observed this happening. It's incredibly frequent right now... I'd say about a 20% chance.