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<p>On airlines where seats are normally pre-assigned, passengers without pre-assigned seats are the most likely to be IDB in overbooking situations.</p>
<p>^That makes sense, but I can’t understand why the website wouldn’t assign us seats when we bought the tickets well in advance of the trip. And before I wised up and got assertive, I called and was told, “Oh, don’t worry, you will get your seat at the airport. We can’t assign you the seat at this time.” I just thought it was very odd.</p>
<p>Probably because the flight was already close to full or overbooked when you booked the ticket. If you can, check the seat map before purchasing.</p>
<p>They do stop giving seat pre-assignments before the flight is full, in order to have a reserve of unassigned seats so that later booking groups with small children or people who need assistance can be seated together. So it is possible that the flight is not really full, and you will get a seat when you check in. But it is also possible that the flight is overbooked, leaving you at risk of IDB.</p>
<p>Note also that some seats like United Economy Plus can only be pre-assigned by frequent flyer elites or people who pay the extra fee for that. But those without pre-assigned seats may be given those seats at check in time when they are the only economy seats left.</p>