The first time Ruth has a convulsion results from a nightmare she has about Mr. Mayo. To make money, Beatrice cares for the elderly (Nanny) and the sick. Evidently, Ruth has epilepsy and her seizures can be triggered by unpleasant thoughts. Mr. Mayo could have been quite sick and thus, he may have physically looked very ill. Note that Beatrice refers to Nanny as a corpse. These types of images just add ideas of death and decay to the hostile lifestyle in which they live. Because she is so bitter, Beatrice doesn't consider what effects the situation will have on her daughters. Here, Ruth shows a tendency she inherits from her mother: reliving past trauma, for Ruth, in the form of nightmares.
Ruth's second seizure is brought on when she finds that her mother has killed Peter, the rabbit. In both cases, Ruth's convulsions are brought on by stress related to the turbulent environment she lives in: their home.