C. F. Keil and Franz Delitzsch on Isaiah 21:8

Isa 21:8

At length the procession has vanished; he sees nothing and hears nothing, and is seized with impatience. “Then he cried with lion’s voice, Upon the watch-tower, O Lord, I stand continually by day, and upon my watch I keep my stand all the nights.” He loses all his patience, and growls as if he were a lion (compare Rev 10:3), with the same dull, angry sound, the same long, deep breath out of full lungs, complaining to God that he has to stand so long at his post without seeing anything, except that inexplicable procession that has now vanished away.