22 As they began to sing and praise, the LORD set ambushes against the men of Ammon and Moab and Mount Seir who were invading Judah, and they were defeated.

Read 2 Chronicles 20:22 Using Other Translations

And when they began to sing and to praise, the LORD set ambushments against the children of Ammon, Moab, and mount Seir, which were come against Judah; and they were smitten.
And when they began to sing and praise, the LORD set an ambush against the men of Ammon, Moab, and Mount Seir, who had come against Judah, so that they were routed.
At the very moment they began to sing and give praise, the LORD caused the armies of Ammon, Moab, and Mount Seir to start fighting among themselves.