Let me preface this by saying I am NOT saying that this was craezie's circumstance.

But what about this situation:

1) 2 adults with 2 young children book a flight
2) only way to get seats together is to pay for preferred seats
3) they decide not to pay, even though they are available because
4) they know the airline will move people to accomodate them
5) so when they get to the flight, that's what happens
6) even though it means people who reserved or paid for preferred seats get moved
7) even though the family could have booked seats together, at a cost, and not inconvenienced anyone.

Now I am not saying this happens a lot (or even at all) but it is certainly conceivable.

as an aside, airlines do block seats to conform with regs but sometimes demand > capacity.