The demon commander wasn't a fool.
Just based on the momentum of the battle, he could tell that they were slowly being pushed back by the humans and elves.
At the same time, there were other humans and elves who were attacking them from the side, slowly whittling their forces away.
If they continued like this, it would be a matter of time before they were choked out and taken down.
So he had to find a way out of this blockade.
In the mind of the demon commander, there were two ways to break free of this blockade.
One was to concentrate his forces and hit one of the weak spots of the blockade. Once they broke through, they would be able to run away and then regroup before deciding on what to do.
But this idea was immediately ignored by the demon commander.
After all, no matter how one put it, this was retreating and it would hurt his pride if he did anything like that.