When you use a spawn egg to spawn a mob (such as zombie pigman) on a floating flat surface that consists of one layer in the top of the nether(above bedrock) ,the mob does not spawn with its feet on the surface,it spawns inside the layer and it falls through it to the bedrock layer.

How to make it happens:

1-go to the top of nether
2-go up at least five blocks.
3-build a floating flat surface that consists of one layer(like creating a superflat world that consists of one layer) from any solid blocks(such as dirt).
4-use a spawn egg on that layer and then the mob spawns and fell through it.