I figured out what happened. I had an EyeTV cache filling up the drive with live video (the video was running on a second monitor and the monitor was turned off.)
So as the video stream filled the drive, SuperDuper just kept backing up the constantly growing cache.
I usually don't have video running when I'm backing up and when I quit EyeTV, it clears the cache, so that's why it never showed much of a change during previous backups of that drive.
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