I also noticed that the Hun horde in my M/M ERE campaign behaved in a similar fashion. It was during the beginning of the my campaign when I noticed that the Hun Horde were pacing east and west in the Northern plains of Thracia Province for at least 20 turns(Constantinople.) Eager to remove this threat to my beloved capital (cheesy way if you must,) I sent my assasin in a failed attempt to remove the three-star faction heir/general. To my surprise and horror, in the next turn they quit their pacing, moved south and sieged my capital the turn after that.

I know that the above circumstances may not be exactly the same as what you described, but I thought I'd shared and hoped to contribute to the dicussion.