When I spawn a lot of mobs some of them get shoved into the ground and act like they are in a hole. Why does this happen?
This is a known bug, and is still unresolved in the most recent release (1.7.2). There are a lot of player theories on what "triggers" it (stairs, ladders, trap doors, etc.) but no official consensus on what causes it, and as of right now, no fix for it. Other than manually "digging out" the mob. (https://mojang.atlassian.net/browse/MC-119)