This prophetic oracle likens the cumulative effect of evil to a massive brush fire burning through the forested Judean hills. Just as a wild fire will burn through everything in its path as it is driven by the hot wind, so the experience of the deepest judgments that can come to a people are found simply by God letting sin run its course and allowing it to burn everything down to charred ruins. That is the scene on display here.
Previously in the context, God has promised a future deliverer for His people and often the only thought of Isaiah 9 is to the messianic beauty of Isaiah 9:6-7. But right on the heels of the promise of a deliverer is this promise of swift and merciless judgment. The sin of His people has led to this decree. It would cover all of Israel and Judah (9:8-9). It was primarily motivated by their unwillingness to repent of arrogant pride (9:9). It would lead to destruction in cities and the countryside (9:10). The historic enemies of God's people would join in the judgment like animals in a feeding frenzy (9:11-12).
The results of God's judgment would be that both political leaders and false prophets would be cut off from the people (9:13-16). Not a family or individual person would be spared the sorrow that was to come (9:17). Those caught up in the judgment would selfishly try to fend for themselves, attacking neighbors as part of the consequence of letting sin burn itself out (9:19-21). It is a brutal breakdown of social order.
This is one reason why we should pray for God to keep His people living holy lives. It restrains the burning fire to a degree. When that restraint is taken away, the world goes up in flames. That is the warning given by Isaiah.