Had a similar issue and found out a couple of new iPhones that were wifi’ed to our satellite service were automatically backing up lots and lots of data to iCloud that didn’t need to be backed up. Changing the phones settings and taking the phones off wifi cured the issue for us. But yeah, we were going thru a month’s worth of data plan in no time at all until the settings were changed.
This might be part of the problem. I do have iCloud shut off on my iPad.