I have also had reserved, confirmed seats changed on me. In one case, I had called to reconfirm the day before the flight, and was told we had 4 seats together on the plane - at the time, this was for myself and my husband and our 2 young children, plus one lap child (under 2). When we got to the airport and checked in, our seats had been changed - they were 4 single seats, scattered throughout the plane.
We haven't flown that particular airline since, and barring some unforeseen circumstance, never will again. But, my point is, sometimes the airlines do things with no rhyme or reason, and messing with seat assignments is one of them.