When you back up to a network volume, Time Machine makes a sparse bundle folder. There aren't any large changes between 10.11 and 10.10 so I doubt the upgrade caused anything. It's likely that you are seeing the band files for the first time or something coincidental to and not caused by the upgrade happened to change the files.
I would open the folder in Finder and see if you can click on the bundle to mount it. Look in Console.app for any errors in mounting the volume or use Disk Utility to verify it. If you are looking at the files in terminal or windows - here is what you should see inside the sparse bundle if things are working correctly and there is no corruption:
mac:~ bmike$ ls -laeO /Volumes/TimeMachineBackups/Mac.sparsebundle
total 208
drwx------@ 1 mike staff - 16384 Nov 10 08:16 .
drwx------ 1 mike staff - 16384 Nov 10 08:50 ..
-rw-r--r-- 1 mike staff - 499 Sep 14 14:06 Info.bckup
-rwx------ 1 mike staff - 499 Sep 14 14:06 Info.plist
drwx------ 1 mike staff - 16384 Nov 10 07:09 bands
-rw-r--r-- 1 mike staff - 512 Nov 10 07:58 com.apple.TimeMachine.MachineID.bckup
-rw-r--r-- 1 mike staff - 512 Nov 10 07:58 com.apple.TimeMachine.MachineID.plist
-rw-r--r-- 1 mike staff - 1302 Nov 10 08:16 com.apple.TimeMachine.Results.plist
-rw-r--r-- 1 mike staff - 13523 Nov 10 08:16 com.apple.TimeMachine.SnapshotHistory.plist
-rwx------ 1 mike staff - 0 Sep 14 14:06 token
mac:~ bmike$ ls -1 /Volumes/TimeMachineBackups/Mac.sparsebundle/bands/ | wc -l
21276
The ls
flags are lower case L, lower case A and E, capital O in the first command and the digit 1 in the second. As you can see, the band files should number between 20k and 40k files of odd names to contain the actual filesystem that stores all the backup data.
