I will shake the heavens (comp. Joel 3:16; Haggai 2:7; Matthew 24:29). In general, this sign is mentioned in connection with the end of the world, when a "new heaven and a new earth" are to supersede the old (Isaiah 65:17; Isaiah 66:22; Revelation 21:1).
Isaiah may, perhaps, pass here from signs connected with the fall of Babylon to those which will announce the last day—each "day of the Lord" being, as already observed, a type of the final and great day (see the comment on verse 6).
Or, possibly, the allusion may be to some "shaking" by God of a supra-mundane kingdom as preliminary to his passing judgment on Babylon (so Dr. Kay; comp. Isaiah 24:21).