I'm wondering if I'm about to get booted from the flight I just booked. Looking around for a nonstop SFO-PHL on 12/19, I picked out US 964 yesterday, but when I went to book it today, it was unavailable on US's website as well as UA's--only connecting flights were offered. On a whim, I tried Orbitz, and there it was! So I booked on Orbitz. Unfortunately, I couldn't assign a seat for that flight--"the airline is assigning seats at the airport" was the message I got.

Is there a meaningful risk that I get bumped? I assume I will be stuck in a middle seat, but is there some way I can avoid this? I've never had such a situation. I have no status on US.