There's no one-size-fits-all answer to this question, as the reason could vary depending on individual circumstances. However, one possibility is that the person who created the Google+ account may have inadvertently added the email addresses of their entire family to the account's "Circles" (or "Contacts"). When a post is shared on Google+, the platform gives the user the option to select which Circles they want to share the post with. If the default selection is set to "Public", then the post will be shared with everyone who has the person's email address in their Contacts. To prevent this from happening in the future, the user can change the default selection to "Only You" under the "Privacy and Safety" settings in their Google+ account.
