Why is Cassie so upset when she's churning butter?

Cassie was upset because of what happened with the bus and the visit from the night men.

I was on the verge of blurting out the awful truth about the bus and the men in the night, but then I remembered the pact Stacey had made us all swear to when I had told him, Christopher-John, and Little Man about the caravan and I said instead, 'No, ma'am,' and began to churn again.