When you unmount cleanly (e.g. eject a drive, rather than just unplugging it hot), you're letting the OS do clean up activity to the drive (which prevents the repair dialog you sometimes see when you plug in a drive).
In this case, the drives are probably spinning up to close file handles, write out journaling data if needed, parking the heads, flushing write cache, etc.
As for why Disk Utility locks while doing this, that's a question only the designers/implementers of the program can answer. My best guess would be that they made the update of the UI an atomic operation with respect to the disk, so that an accurate state is always reflected in the UI. If they background this kind of operation, the UI is indeterminate with respect to the actual state of the disk.
It would also require some guessing as to what operations are safe to background and which are not.