[Bug] Tremor only affects one unit?

I had a horde of rock spiders approaching one of my cities, so I shot them with Tremor to lock them and weaken them a bit (- Defense) while my army was healing up in town for one extra turn.  When I engaged only the head rock spider had a tremor effect on him in tactical combat, all of the little spiders did not.  Since the spell is not resistable I would have expected every unit in the enemy army to be @ -4 Defense, not just the enemy stack commander.

- Manii Names