Both Arachne and Fenrir were assertive in their intentions to finish off the fight before Noah arrived. Their tunnel vision for quick victory made them unable to realize that Noah had been there, watching them all along.
On his way there, his mood soured with every step he took. The distance he had already covered showed that the supposed enemy was not even remotely close to their temporary base, and yet the two still decided on their own to engage the horde. His jaw clenched in frustration. They acted without his approval, it wasn't that he felt his authority was being challenged, he was angered because of their recklessness.
