why does the stranger say she is afraid of folks

Sylvia is afraid of the sound of 'a boy's whistle, determined, and somewhat aggressive.' She has enjoyed the peace and comfort of the forest and believes the human sound is threatening - ' the enemy had discovered her.'

It is indicated by Sylvia's grandmother, Mrs Tilley, that one of the reasons she brought Sylvia to live with her is that she was considered 'afraid of folks' when she lived in town.