Are you 1K? It's happened to me before when booking high-volume business flights on short notice (e.g., booking a Monday 6am ORD-LGA flight less than a week in advance). I've not been involuntarily bumped yet. There are almost always no-shows and as upgrades get processed, seats open up. Also, they do tend to hold a few seats until 24 hours (?) prior to departure, like some of the bulkheads (I assume for those potential special needs passengers).

Your seat might not be the best, but you will likely get a seat.