Multiple Mobs spawning

I was simply traveling from the Nether back to the overworld and when I got the whole world was covered with Creepers that kept spawning every second. This happened a second time but instead of Creepers rapidly spawning, Endermen kept spawning.

What I expected to happen was...:
I expected it appear in the overworld like normal.

What actually happened was...:
One time the map was covered in creepers rapidly spawning, the other time in Endermen rapidly spawning.

Steps to Reproduce:
I am unsure how to trigger this bug as it has only happened twice so far out of all the times I visited the Nether and was completely random.