Why does this fleet retreat?

[StarJack]

The fleets have an arrowhead formation, and a strategy of optimal firing range. They went threw the system behind them blasting planets and small fleets with no problem. Got to their current system, retreat for no apparent reason. I sent them back to resupply fully, send them back to this system, they retreat for no reason. I looked under optimal firing range strategies and do not see anything that would cause the fleet to suddenly start retreating, I don't see any 'empty' or 'null' entries, not sure if you meant literally?

Oh, and no fighters involved but I have plenty of PD. The weird thing is the fleet was going along just fine and then, pffffft..