Update: I just went through my mail in Outlook and deleted the huge pile of messages sent back and forth regarding the condo purchase, getting the closing scheduled, etc. I suspected that if a particular item (or items) was the source of the problem, it would be one of those - since they all originated at the time I was using webmail daily in order to keep up with what was going on while I was away from my home computer.
Happy to say that as soon as all those items were trashed, the problem disappeared. I've closed and reopened Outlook and there's been no recurrence of the issue. We'll see if it keeps behaving but I believe it was probably my moving items and folders around in both applications that was the culprit and getting rid of those items completely did the trick.
