Server notifications gone berserk. Cleaning up old tags, seeing as Gmail is so very fond of encouraging archival instead of deletion.

I've developed habits of using mail for all kinds of notifications because it tends to work with other services, and it mostly works with Gmail too, exception deletion it a totally pathological case. All they'd needed to do would be to queue large operations in the background...

Testing/QA accounts are one use-case where you can get a ton of mail.