Actually, not all Pfhor do this, only certain ones. In a Marathon physics model, each monster/alien can be tagged as a 'Berserker' (located under 'Behavior Settings'). A monster with this tag turned on will show the behavior you've probably been noticing, which is to say, when significantly damaged, 'Berserker' monsters will attack anything in sight, regardless of its normal Friend-or-Foe settings.
As a general rule, only Major Pfhor have this tag actually turned on in the game. So, green and orange Fighters will not go berserk, but purple and blue ones will. This can be quite useful, as it allows you to let a whole group of enemies annihilate each-other while you simply wait around the corner listening or watching to determine when the killing has stopped.