The last time TM was backing up it did not dismount the sparsebundle correctly.. and it finds that it is still mounted.
It means you need to kick your TC.. figuratively..
Make sure the disk is not spinning..
(you can feel the vibration with your hand on it)..
Pull out the power cord.. count to 10.. push in the power cord.
That fixes 98% of the problems with TC nowadays. Or it seems to..
Look at C12 if you are in the 2%