When mining blocks that have adjacent non-transparent blocks (i.e. stone, cobblestone, dirt etc.), the entities can glitch into the blocks and thus rise to the surface. It happens around 8-10% of the time. I surface after caving to find redstone ore, iron ore, gold ore etc. all over the ground.
Environment
Mac OS X 10.9 running minecraft_server.14w08a.jar (no plugins, pure vanilla)
Yes, I've noticed this very often too. It is still an issue as of 14w10b.