Cannot turn off iCloud backup

Hi there. I have an iPad Air 2, running the latest update of iOS (9.3.2). I'm having an issue with iCloud backup. I periodically get a message saying my iCloud storage is nearly full, which I find odd, since I don't use it for much beyond email and some document syncing. So I investigated in the settings app and discovered my iPad is backing up to iCloud (2.9GB or so). I always prefer to backup through iTunes to my iMac (to avoid paying for more storage), so I delete the backup, which notifies me that it will delete the backup and turn of iCloud backup. Which is exactly what I want. But two weeks later, I get the same message and find out that it's backed up to iCloud again.

Has anyone run into this?