I doubt the cars were actually sold, leases probably expired and went back to leasing company marks. AOK seems to be a favorite reporting mark for at least one leasing company to use, another is NOKL. Don't know which leasing firms use which marks. Rolling stock and locomotives are mostly on long term leases these days, outright ownership is pretty limited. I do think it may be a red flag that BNSF let the leases expire on its ore gons, but it could mean nothing and that they wanted to improve car quality by getting newer cars, the BNSF cars have been in that service a long time and had been through at least two shoppings during that time---it could just be that the floors are getting thin and they don't want to risk the roadbed contamination issues that the UP had to deal with on its Wallace branch.