Oh, my...I just saw a "lite" version of this at a wedding last night!!
DH and I were both in the bridal party, so we were late to our table. We found four people there and four empty seats (two of which were ours, of course), and one of the guests mentioned that another place setting had been moved to a neighboring table. Recalling the seating chart I realized that three people who were meant to be seated with us had moved to that next table. There had been two empty seats there (last-minute cancellation due to weather), and the third guest had asked the waitstaff to move her seat so she could go along.
I kind of understood, since DH and I were the "bridge" for the two groups, and we weren't there at first. The shyer group apparently wasn't comfortable being seated with people they didn't know well (I doubt they would have had contact since our wedding, six years ago), so they moved to their family's table. No one was left alone, but I thought it was quite a shame--they basically travelled hours by plane to have dinner with their parents, instead of hanging out with a fun, friendly group they'd been seated with. And that group was a bit put off, as well, because even if the rejection wasn't nearly as brutal as dragon_heart's it was still the sort of thing you can't help but notice.
I certainly hope that moving one's assigned seat isn't becoming an epidemic, because it hardly ever seems to be a good idea.