You can blame me for creating this mystery, but I'll also take credit for solving it.
There is a glitch in the software where the "Thanks" are not removed when a post (or thread) initially get's removed. Threads and posts are sometimes removed if they violate the
forum rules, we receive a complaint or somebody chooses to end their membership with us.
Once or twice a year we clean up the older deleted posts and physically remove them from our servers. At this point, people's "Thanks" counters are updated so "Thanks" that were left in deleted posts are no longer counted. For the past couple of days we have been doing this. We're all done for now so people shouldn't see any more dramatic changes until the next time we repeat the process.
Thanks,
Alex