I suspect it’s an “Advance” reduced fare ticket, which is only ever valid with a seat reservation, but either the seat was over specified (ticking all three of facing forwards, table seat, near the entrance, for example), or the train company continued to issue “Advance” tickets even after all the reservable seats are gone, which you could count as a dick move, or you could interpret as allowing more people to buy tickets at the reduced fare.

It could be that that was one of the least overcrowded trains scheduled on a day that’s expected to be very overcrowded indeed, and they’re trying to spread the no standing room pain across as many trains as possible. It’s certainly cheaper than putting on additional services.