Ghasts can place fire from fireball explosions.

[Anna Bakker]

Exactly what the title says. When a ghast fireball hits something, it lights it on fire, this would most likely happen in the nether and hit netherrack, so the fire won't go out unless it is put out by a player.

Original post: https://bitbucket.org/minecraftonline/scripts/issue/125/ghasts-can-place-fire-from-fireball