Yes, we have seen a few problems that appear to involve the Event Store on 10.6.8 (usually with different symptoms, though).
In some of those cases, the Store was quite large -- speculation was, it was somehow damaged. In most of those cases, we'd see very long backups, with huge counts of files backed-up (tens of thousands), but very little space (sometimes well under 1 GB). A fix seemed to happen with a Safe Boot -- that seems to have found the problem and fixed/cleared out the Event Store; the next backup was a long one (deep traversal), but thereafter it behaved normally.
So it's not beyond the realm of possibility that a reboot finally found and fixed the problem. Just for chuckles, you might want to look at your system.log for the time of the reboot, and see if there are any messages about the Event Store, fsck, or any reference to your internal HD.