I haven't found a sure way to reproduce this effect. It mostly happens when you draw out a single creature from a group and kill it. The remaining creatures of the group appear to be rooted to the spot once you enter their visual range. They will attack however when you're entering their weapon range.
It looks like the creatures are twitching as if they're trying to move, which leads me to believe it may have something to do with the new pathfinding algorithm because occasionally even my party members are stuck on visible or invisible obstacles (like other creatures) forever unless I manually assign new orders.
The attached save can be used to reproduce this issue. A bit to the east of Charname is a group of four shadows. If you lure one of them out and kill it, you can safely enter the field of view of the remaining three shadows without being attacked. Saving and reloading seems to fix this issue temporarily.