[Sheap]

There are many things that can cause independent attacks:

* Random events: Vampires, Knights, Barbarians, Bogus the Troll, Vine Men, Caspar of the Cave (who doesn't attack himself, but unleashes independent monsters), rogue mages. There are different independent attacks underwater. Some of these events are rarer than others.
* Horrors (can be caused by horror mark, or by the Send (Lesser) Horror spell), and Ghost Riders. These disappear after combat.
* Raging Hearts (I believe Imprint Souls is controlled by the caster although not sure)
* Unfettered Eater of the Dead
* While not exactly an attack, there is also a rebellion random event.

Maybe others. This is just what I can think of.