We, Daapo and I, created a TNT canon and we decided to implement a falling anvil, so, with the explosion, the anvil would kill some cows.

What we found out was that the anvil didn't fall if the crowd was too heavy. The anvil floats over the cows (we have not tested more mobs) and then it slows down until it starts teleporting [_source_]. When it reaches something "solid", such as dirt, obsidian..., it moves towards left or right [_source2_]... We tested this "towards positive Z".

What's more, if the anvil moves towards left or right and there is alredy an anvil there, it drops the item.