Mrs. Sappleton appears to be a thoroughly domesticated, uneducated housewife whose life is devoted to her husband and brothers. It would appear that she is trying to train and condition young Vera to become a housewife and hostess just like herself, and that Vera despises her aunt for being such a slave and a doormat. Mrs. Sappleton spends her time waiting on, and waiting for, the menfolk. Even when she is talking to Framton Nuttel, her attention is focused on the open window as she waits for the first glimpse of the returning hunters. Framton naturally thinks this is a sign of her insanity, since the hunters have been dead for three years.
...he was conscious that his hostess was giving him only a fragment of her attention, and her eyes were constantly straying past him to the open window and the lawn beyond.
Vera knows the whole scenario by heart. They are sure to get mud all over the floor and carpets, but Vera's aunt accepts that as a normal part of her daily life. Since the menfolk think about and talk about nothing but shooting birds, Mrs. Sappleton doesn't hear anything of what might be going on in the outside world. She has become conditioned to talk about birds herself, and she has apparently become somewhat knowledgeable on the subject from listening to these boring men. She naturally assumes that Framton, being a male, must be interested in shooting birds.
She rattled on cheerfully about the shooting and the scarcity of birds, and the prospects for duck in the winter.
It would seem that Vera makes up her horror story as a sign of rebellion against the prospect of becoming another empty-headed country homemaker like her aunt. Poor Framton is the innocent victim.