i've seen it happen a few times over the past month. it's not a very large number, just enough to throw the balance out of whack and it only happens when one side is not far from winning the war. last time i saw it happen, 20 or 30 jumped from bish/rook to knits when knits only needed 2 bases to win the map, by that time a good number of bish must have been sent to bed and it ended up something like bish 84, rook 132, knits 168.
nobody was switching to bish for whatever reason...once the map reset a lot of people from rook and knit logged off.