Most common cause I have noticed this on Tasman/pacific flights is when there has been an aircraft change. They tend to take 2-3weeks to update seating allocations and re-seat everyone before they re-open seat select.

Pretty common the Tasman to see an A320 swapped out for a 763/772/77W when there is high demand. Haven't really had a bad seat allocation out of, always end up getting re-allocated a PE seat.