How to prevent calendar alerts from becoming Drafts (???) in Mail.app?

I don't know the rhyme or reason to it yet, but random calendar event notifications in Lion turn into drafts in my Mail.app drafts folder. I usually find these when I wake the machine up after it's been idle for some time.

Why would an alert become a draft in Mail.app at all? It makes no sense.

The calendar items are from a shared Google calendar. The draft is marked "no recipients", and just has the basic info from the calendar entry. The subject is like this:

"Alert - Piano lesson"

and the body is like this:

"September 5, 2012 1:45 PM : Piano lesson"

My wife enters those in HER gmail account calendar but I subscribe to it.

Any ideas?

Calendar invitations are delivered via email, so perhaps Mail app is preparing things for you to "accept, tentative, or decline" the invitations as part of the routine processing of a mail message that gets diverted from the inbox and places in another application (Calendar in this case, or iCal for those on Lion and earlier).

You could also just configure Calendar to not retrieve CalDAV events from Mail in the first place and let mail handle things in the inbox rather than having the system try to process things for you.

Calendar -> Preferences -> Advanced

Deselect the box next to Automatically retrieve CalDAV invitations from Mail