The same idea as in Mat 24:28. Where a corrupt nation is, there God’s instruments of punishment are sure also to be. The lion roars loudly only when he has prey in sight.
Will a young lion cry out ... if he - the “lion,” not the “young lion.”
have taken nothing? - The young lion just weaned lies silent, until the old lion brings the prey near; then the scent rouses him. So, the prophet would not speak against Israel, if God did not reveal to him Israel’s sins as requiring punishment.