Calendar event becomes invite iOS 9
I've been experiencing the exact same issue.  Outlook 2013 which synchronizes with Office 365, I-Phone 6S Plus with IOS 9.1  

If I modify or create a calendar entry on my I-phone, it changes the calendar entry to an invite in Outlook.  In Outlook the calendar entry is shown with a semi-colon and followed by my name.  

If I open the calendar entry, the To... field is displayed empty and an information message: "You haven't sent this meeting invitation yet."

If I click on "Cancel Invitation", the entry becomes a calendar entry again and the Invite disappears...  Until I edit it with the I-Phone.

Hmmm...