‘Now the vale of Siddim was full of bitumen-pits, and the Kings of Sodom and Gomorrah fled, and they fell there, and those who remained fled to the mountain.'
They had probably hoped that the particular site, with its related problems with which they were familiar, would offer them an advantage but they had no chance against a superior force and the two main kings were killed while the remainder fled to safety in the mountains.
We have already been told that Sodom was a wicked city (Genesis 13:13) so it may well be that in the context of the narrative as a whole this is seen as God's preliminary judgment on Sodom.