I just noticed a great deal of activity with my backup disk, and found Time Machine copying some 30 GB+ of files. A routine (~100 MB) backup had just occurred an hour before, and I hadn't made any major changes, or done much beyond answering a few emails. When the backup was done, I launched TimeTracker to see what had been backed up. It turned out to be more or less my entire Applications folder, plus about 7 GB from my root-level Library folder, and about 3.5 GB from the System folder, and a bunch of other assorted folders. Here's a screenshot from TimeTracker showing what was backed up in that session:
The only thing I think may have happened before that backup was installing a minor iTunes update (a fairly routine occurrance).
I'm not particularly worried about the extra space being taken up on my backup drive, but I want to make sure this redundant backup doesn't point to any more serious problem.
I should point out that I'm running 10.8.2 and backing up to a LaCie d2 connected via FireWire.