Random ghasts fireball

I was walking in the Nether and taking screenshot of the light bugs (MC-3329) when I saw that was something strange with the fire. Then I understood that wasn't netherrack burning, but Ghast fireballs still in the air.

The problem is: there were no Ghasts to shoot them, because I turned the difficult to peaceful >before entering in the portal<. This makes this issue different from MC-27.