They do attack the AI factions. First time I played Egypt, when the Mongols arrived, they attacked the Turks first. Though oddly enough, Mosul rebelled on the Turks during the siege, and the siege was lifted. Didn't take them long to retarget themselves on me that time. All the otehr times, they tended to settle with other factions. My current one as the Byzantines, they originally took the Kieve region from the Poles, but the Poles managed to take it back and drive the Mongols back into wandering. Now several stacks did reappear during the northern borders of the Turkish lands, but they ended up wandering toward Kiev once more. Only time they attacked me was during a Jihad, and I assassinated the leader leading the Jihad army, making them go away (oh I so love doing that).