I've seen some other people report something similar, but not quite what I have experienced.

I've been noticing that if a passive Enderman (not aggravated by the player) takes fire damage not inflicted by the player (fire, lava, etc.), then the Enderman in question will immediately become hostile and attack the nearest player.

So far I have only seen this occur uncommonly in the Nether, but it is extremely annoying too as the Enderman (in addition to the damage it already deals) will deal fire damage to the player, which can be quite dangerous or even fatal when in the Nether.

I suggest that the Endermen should react to natural fire damage as they would with rain (that is, quickly teleporting around the area). This will prevent both unprovoked attacks on the player, and other players from exploiting fire damage on Endermen to farm pearls (as they will be hard to keep up with).