it actually sends the email? as in, the recipient address receives the incomplete email?
otherwise, the draft email is being uploaded while you're composing it.. for example, you can start a mail on computer A and finish it at a later time on a different computer/device.
if you do a bunch of deletes then your mail is being edited and the changes will reflect in the draft (and the draft is likely being stored on the cloud).. which is why its possible you're seeing activity when deleting.
that said, if the mail is truly sent to the recipient then i don't know what may be causing this to happen upon using rapid delete key presses.