What could be the cause of this? The only thing I can think of would be mobile Time Machine backups, but I have no idea.
I think you might be on to something here, the .MobileBackups folder is supposed to be invisible, but sometimes does actually take up storage space. The other thing is caches, they can easily grow to gigabytes in size, though I wouldn't expect them to take up 100 GB by themselves...
Whatever the case, if you still have your files in your home folder, then you're probably good! But I hope you made a backup before installing the update anyways, right?!
Mac OS X allocates swap space in the file system (in /private/var/vm – don’t touch these files!) and when the system reboots it reclaims that space. If you’ve been running programs with big memory footprints they could have consumed a lot of disk by virtue of that and it would only be released when the system restarted after upgrading. It may not have been the upgrade, it may just have been the restart that caused the disk space to be released.