Are these difficulty drops related to dying and reloading? Some time ago, in
this topic, Senior Cinco reported that issue. It isn't very common, though, and I don't know if there is a solution other than manually changing the difficulty (you can try emailing support@larian.com, with a description of the problem and maybe a link to this topic).
As Joram mentioned, there have been a few cases where certain opponents were either very tough or could not be harmed, I think more in the original release of D2:ED than DKS (generally leaving the area and returning would fix the problem). In D2:ED one person reported that boars could not be hurt with magic, until they were hit with a weapon (many of the boars/ducks/chickens were removed in DKS).
I don't think the difficulty level for the fight with Zagan is a bug/glitch, though, since a couple people reported it was challenging even on casual.
I ran into two cases with particular groups of opponents being much more difficult than they should have been (on nightmare). The second area I died and reloaded a few times, quit to the main menu, then decided to give it another shot and loaded the auto-save, after which the game made the difficulty much lower (with the difficulty setting unchanged). In both cases the difficulty was back to usual after restarting the game the next day (I also played the Xbox version).
There are no difficulty level based achievements.
