Selfish Begets Selfish
While Faye and the others had found the river, there was also another batch of people from the Rows village who went the other way upstream.
The village was not small and things happened quickly that people just went in different directions. However, since they had access to streams, they'd go just one way or another.
Another few hundred people went in this direction. They left the moment they saw the buildings disintegrating.
While most had the instinct to wait for the leading party to start moving—particularly because they had been dependable so far—some had rushed out earlier when they saw the reaction was delayed.
Some had chosen the direction by random, while others did have reason for going upstream. For instance, at the very least, they knew there was really a mountain in this direction. It was more difficult to traverse because there was a slight incline, but at least it was better than hoping to find a river before dusk.