You only find it by snooping and reading journal's but the Grove was almost entirely out of food, they literally couldent support the trifling any longer.
All the while during this Halsin is experimenting in secret on the dead drow, and decides to leave for the ruins, caring nothing about the empty food stores
The no-food angle sounds like great "meat" for the story. Kagha would be 100x more reasonable if she was struggling to feed everyone. "We can't go outside the grove to look for food because of the goblins, and everyone here will starve if these refugees stay. They keep saying they are about to leave. Always about to leave. Are they going to wait until every scrap of food is gone?"
Kagha should also play up more that Halsin abandoned them. She doesn't want to make these hard decisions, but Halsin left her no choice.