Why can’t I retreat?

Playing as Gorbad I attacked Thorgrim around turn 10-15. I have a reinforcing waghh army. Plus a lord with its own extra army. I figured I’d steamroll the dwarves. But VH strikes with an insane couple armies. So I besieged them tried to attack an army. He retreated but managed to stay in reinforcing range. Which in itself is BS. Then over the end turn I get this. And it won’t let me retreat. I might be able to win. But what is causing this inability to fall back?

Playing as Gorbad I attacked Thorgrim around turn 10-15. I have a reinforcing waghh army. Plus a lord with its own extra army. I figured I’d steamroll the dwarves. But VH strikes with an insane couple armies. So I besieged them tried to attack an army. He retreated but managed to stay in reinforcing range. Which in itself is BS. Then over the end turn I get this. And it won’t let me retreat. I might be able to win. But what is causing this inability to fall back?