Honorable foes

Although the enemies may surround the player, they refuse to attack him or her together, instead, only those in front of the player attack or possibly only one of them. Sometimes the surrounding enemies may perform an attack, but this is nothing compared to those in front of the player.

This may also apply NPCs fighting other NPCs where they refuse to attack from any other direction than front of their target.

Shooting enemies do not usually follow this rule even when their melee friends do.

Note that this isn't a bug (except in case of very peculiar AI code), more like a feature added to make fights less hard.