btrfs used to have this issue, the problem being that the filesystem has to append metadata to do any operation including (ironically) deletion: https://serverfault.com/a/478742.

AFAIK it's fixed now, because btrfs reserves some space and reports "disk full" before it's reached. macOS probably does the same (I'd hope), but it seems in this case the boundary wasn't enforced properly and the background snapshot caused it to write beyond.