I've seen a weak army sacrifice itself and spring my trap that I had intended for another unit just to piss me off!! lol That army had other options of where to move but headed straight for my trap.
The unit I actually wanted to hit the trap had the next turn and the AI knew this so sacrificed a shitty army to spoil my plan. By doing this the army died and the much larger unit did some big damage to my unit that was close to it, if that makes sense.
In any case, I thought it was quite clever of the AI