^18:8 For this reason in one day her plagues will come, ^[a]plague and mourning and famine, and she will be burned up with fire; for the Lord God who judges her is strong.

For this reason in one day her plagues will come, ^[a]plague and mourning and famine,

The reason is the Babylonian pride that visibly shows itself in glorifying oneself (previous verse). Self becomes the idol or god and this is the quickest way to "irritate" God (Exodus 20:1-5-6).

In the case of Belshazzar, as Daniel saw this in Daniel 5:17-30-31, the judgment happened very quickly later that night. So, too, this judgment happens "in one day". Babylon will be destroyed as fast as Sodom and Gomorrah was destroyed according to Isaiah 13:17-22.

Combine these weaknesses and death results both physically and spiritually.

and she will be burned up with fire;

Anything which turns us away from God will be tested (1 Corinthians 3:10-13-15).

Anything related to the pride/glorification will be consumed (Hebrews 12:18-29).

Anything seen by Jesus Christ will be judged (Revelation 1:14).

for the Lord God who judges her is strong.

Many times Babylon is referred to as GREAT (verses 2, 10, 19, 21) and thus implying STRONG. But the LORD GOD IS STRONG eternally (implying greater than Babylon). The LORD GOD will complete what was started in the seventh trumpet (Revelation 11:15-18).

^aLit death; i.e., a particular kind of death