When mining blocks, the mined item that pops out can clip into the block above it (if one exists); The item is then shot upwards through the blocks until it finds a non-solid block (air/water/lava/etc) and pops out. Usually this results in the item(s) arriving (somewhere) on the surface, or becoming lost in some gap (or in lava) above where it was mined.

This seems to happen more frequently when mining a block that has more neighboring blocks around it. (ie: Mining a coal block from underneath, that has 8 surrounding blocks + the 3x3 layer directly above it.)