Modern (well, post-ZFS) filesystems operate by moving the filesystem through state changes where data is not (immediately) destroyed, but older versions of the data are still available for various purposes. Similar to an ACID-compliant database, something like a backup or recovery process can still access older snapshots of the filesystem, for various values of "older" that might range from milliseconds to seconds to years.
With that in mind, you can see how we get in a scenario where deleting a file will require a minor bit of storage for recordkeeping the old and new states, before it can actually free up the storage by releasing the old state. There is supposed to be an escape hatch for getting yourself out of a situation where there isn't even enough storage for this little bit of record keeping, but either the author didn't know whatever trick is needed or the filesystem code wasn't well-behaved in this area (it's a corner-case that isn't often tested).
With that in mind, you can see how we get in a scenario where deleting a file will require a minor bit of storage for recordkeeping the old and new states, before it can actually free up the storage by releasing the old state. There is supposed to be an escape hatch for getting yourself out of a situation where there isn't even enough storage for this little bit of record keeping, but either the author didn't know whatever trick is needed or the filesystem code wasn't well-behaved in this area (it's a corner-case that isn't often tested).