I had a terrible experience as well a few days ago. Using iPhone 6plus with IOS 11 (11.0.3)
I had received a calendar invite a few months ago with the heading "Lunch with xxx". It had 27 invitee's. I clicked on noon of the current day to book a new appointment, typed "Lunch" as the subject and added one of the recipients that happened to be in that other older unrelated invite.
I suspect a vombination of the above Siri features combined with new Siri IOS 11 features (Type to Siri, Siri Suggestions, Siri Search) which are now integrated into applications.
(sidebar terrible invasion of privacy by Apple, it leverages some search data across all apps. See Settings->Siri&Search->"About Ask Siri & Privacy" Link.)
With the interface for calendar items the way it is, its not obvious except for the little "27" next to invitees that the message is going to go to all those recipients. The one thing I know and remember for sure is that I only added one name, Siri did the rest... Terrible feature, abuse in my opinion. There is no need unless I EXPLICITLY want to enable it.
I have disables both siri search and siri lookup and disabled it in each application. See Settings->Siri&Search.
Have a look at this related issue https://arstechnica.com/gadgets/2017/06/ios-11-will-bring-big-updates-to-siri-im essages-apple-pay-and-more/
Apple, assume users dont want to have thiert privacy rights diminished even if you think your doing us a favor. If you want to help, never enable by default, put the TOU in a smart place and MAKE IT OBVIOUS in SIMPLE ENGLISH so mom and pop can understand what they are in for.