"I suppose it'd be enough to just say that they felt like something was about to happen, I only ask out of curiosity because I don't think you mentioned why."

Due to the tribes high animosity with each other, they would naturally spy and raid on each other. After having witnessed what happened to the Mutig... They would naturally fear the same was going to happen to them. And if none of their scouts made it back, they'd fear the same thing. This was vaguely hinted at when the champion mentioned the Mutig.