because they can
Seriously, they have probably changed the version of the 747 flying the flight you are on and when this happens the seat REQUESTS (not a confirmation or allocation, its only a request) are removed.
Secondly, if the flight is full and the 60% of seats held back have been released, then only a manual change to seat allocations can be carried out at teh airport and you get a message that says "Seats will be allocated at checkin"
Which flight are you on