Ooooh, don't get me started how mad it makes me! I am paranoid (or just lucky) enough that I didn't yet had it happen to me, but my wife's phone had done it four times in the last year. Each time I check and double check that all "backups" are turned off, and each time it somehow pops back.
So, Google "backs up" a 128Gb worth of photos on the phone onto 15Gb free storage combined with Gmail and who knows what, completely clogs it (as if it couldn't be predicted) and then has audacity to suggest paying for "extra storage". There is no way in online UI to just delete the whole "backup". And the cherry on top: when you finally get to delete some there is a fine-print - "the selected photos will be deleted from all synced devices". Well, I guess I must be thankful that they at least show this warning. This is what passes as "backup" in Google's parlance these days.