[Suicide Junkie]

From the name, it sounds like the AI is probably breaking formation because the leader can't move.

If the AI ever decides that the leader is stuck, it will break up the fleet so that it can move the ships.

Before that, you could sometimes defeat a large enemy fleet with the proper manouevering of a quick escort, causing the enemy wingmen to box in the leader, and bringing their advance to a halt. Wait out the 30 turns, and then counter attack with a large defense force.