Overworld mobs spawning in the Nether

I have been testing a mob farm in the Nether that I originally built in 1.5.
After making a few changes I was going to test it again and to my surprise it got filled with mobs from the Overworld, zombies, endermen, witches and even chickens! Also, I have not seen any ghasts for a while...

I tried it both in creative and survival.