Probably because it's a log-structured file system, and those _really_ don't like running low on free space. They work by appending to the log then compacting

Probably because it's a log-structured file system, and those _really_ don't like running low on free space. They work by appending to the log then compacting sometime later, not modifying things in-place. As such, you always need a reasonable supply of free blocks so this can occur.