The Bug:

When entering the new biome, Soul Sand Valley in the nether or created a buffet world of the biome, Ghast will start spawning at an alarming rate.

I used buffet world to reproduce the bug easier.

To reproduce:

1. Create a buffet world in "More World Options" and customize it to Soul Sand Valley.
2. Create the world and let it load in.
3. ->  In around 3 - 4 seconds, about 30-35 Ghasts spawns in at alarming rate.

It might be intended though, as this could be the homeland for Ghasts.