Albert Barnes on Revelation 8:6

And the seven angels which had the seven trumpets prepared themselves to sound. See also Rev 8:7. Evidently in succession, perhaps by arranging themselves in the order in which they were to sound. The way is now prepared for the sounding of the trumpets, and for the fearful commotions and changes which would be indicated by that. The last seal is opened; heaven stands in suspense to know what is to be disclosed; the saints, filled with solicitude, have offered their prayers; the censer of coals has been cast to the earth, as if these judgments could be no longer stayed by prayer; and the angels prepare to sound the trumpets indicative of what is to occur.