There are a number of write behind caches associated with the file system, and some other subsystems. These vary by the amount of changes, the type of changes, and the time since previous sync, most systems are configured to sync at minute intervals, but that is configured differently based on the distro. Certain types of writes are cached until a little bit later to try to combine sequential writes and other things. It makes a huge difference on system performance. Anyway it is a write behind cache that needs to be written at such times. On larger systems it can take minutes to complete these items.