Mobs killed by environmental damage do not drop experience

If a group of aggravated zombie pigmen is lured in to a death trap and dies from fall damage, they should all drop their xp. But it seems that only the one that initiated the assault (the one that got hit by a player) are dropping xp.

How to reproduce:
On a platform high above ground, spawn in many zombie pigmen.
Put the player on another platform, with a bridge of open trapdoors in between, not longer than two blocks.
The player (in survival mode) shoot one pigman with a bow.