For a while (not sure if it's still true) if you went through the united.com booking flow and didn't select a seat it would just arbitrarily assign a seat assuming one you were eligible for was available... I remember once maybe 3 years ago when the only seats available in F were aisles -- I didn't particularly want any of those so figured I'd let departure roulette figure it out for me at check in (assuming a window didn't open before hand) -- but once completed checkout and looked at the PNR I had been auto-assigned to 3D or something like that.