Final answer:
The actual event causing Henrietta to start shaking and cackling is not detailed within the provided excerpts, but such actions typically signify a major plot twist or character's emotional peak in literature.
Step-by-step explanation:
The surprise that happened when Henrietta started shaking and cackling isn’t specified in the provided excerpts. However, these types of actions in literature usually indicate a dramatic revelation or a critical moment for a character. For example, a character suddenly realizing the truth behind a situation or undergoing an intense emotional response could be reasons for such behavior. The text snippets provided suggest high tension and emotional moments but do not directly relate to Henrietta or explain her specific actions.