I could be wrong, but it appears that the AI learned this strategy from me. Suddenly it was not targeting just the first aircraft, it targeted each individual aircraft partway through the combat.
Edit: This was apparently a fluke, as repeated tests involve the enemy aircraft following only the first aircraft again.
Edit: I think I figured out what makes the difference. When I engage the combat (evidenced by the box popping up and giving the option to tail, engage, etc.) The enemy aircraft do not seem to all target one of mine. If I let the enemy initiate the combat, they all target my first aircraft.