Welcome to Android Central! Is it possible that you might have done this by accident? Were they compromising or embarrassing photos, and was there a specific message sent with the Google Photos sharing? If not, I can imagine that a person could relatively easily tap the Share button while looking at a photo, then tap the Google Photos option for a specific contact (especially if you had shared photos with that person in the past). This kind of thing could potentially happen with unintentional touches, like when the phone is in a purse or pocket.