Obviously I don't know what happened here, but this scenario is quite common. Seats have been sold but not assigned yet. We have a skewed perspective at here FT; many normal people don't fuss over such issued. (Some family members recently went on their first Europe trip, and it was a struggle for me to convince them to pre-select good seats. They didn't think it was a big deal and didn't understand why anybody would fuss over something so trivial.)

Another possibility, many airlines make it quite difficult to pre-select your seat unless you are an elite or pay extra. Again, not sure if this was the case here, but it happens.