Yes, I too thought that a few odd things were happening yesterday, even before I noticed these new people appearing (by the way, the new people are correct, more or less).
Regarding the particular conundrum of my mystery adder-of-people, I realized that although the tree wasn't shared with anybody there were three outstanding invitations to share my tree as a guest. So I cancelled them just in case. But I got an email late last night from a contact that seemed to indicate that one of those people, who turns out to be related to my contact and is actively researching the family in question at Ancestry, did actually receive my invitation. So perhaps the invitation was accepted and perhaps he added the new people?
Except that:
a) Ancestry didn't inform me, either via a 'Your invitation has been accepted' email or via the Tree Settings / Sharing option in my Ancestry account, that the invitation had been accepted.
b) he was invited as a guest, and guests can only view the tree, not modify it. I've just confirmed that for myself.
I'm currently awaiting a response from my contact to confirm whether or not the invitation was accepted, and whether or not the invitee actually made changes.