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.