Time Machine sparsebundle corrupted on disconnect
Sadly this is why the Airport Extreme isn't supported for time machine. It doesn't have the smarts to repair the filesystem or handle a disconnect as reliably.  This isn't an issue with time capsules or a server than can clean up when the backup gets interrupted. 

You will have to nurse your setup as described, but the lifespan of sparse bundles on airport as USB disk are limited. 

Perhaps a firmware update or change to Mac OS X will make things better, but for now it's something to watch.