As far as I can tell, all enemy take extra damage when hit from behind. This is very notable early game when you have to hit things more than once usually but when it runs off like an idiot (looking at YOU, grey dwarves) and you can get that hit on its back it will go down in one hit instead of 3. Most other enemy only turn their backs if exploited by the terrain, so its hard to manage.
I don't have a for sure answer to your question, but it should be easy enough to test.