The phrase speaks to the suddenness (and hence unexpectedness) of the disasters coming upon Babylon (Isaiah 47:9; Jeremiah 50:31). More, the phrase speaks to the direct involvement of God’s mighty hand (consider how the plagues of Egypt were miracles from God coming suddenly on the land, Exodus 7—12. The notion of suddenness recurs in Revelation 18:10, Revelation 18:17, and Revelation 18:19.
8 For this reason her plagues will come in a single day, death and mourning and famine, and she will be burned up with fire; for mighty is the Lord God who has judged her.”