Some flights from certain parts of the South and West are targeted for immigration control checks on arrival when there is suspicion a flight has been used for or has a passenger that has evaded US immigration controls. It's very infrequently observed -- as it impacts so very few flights historically -- but it's part of the arsenal used by DHS rather routinely at that. I've more frequently encountered this on departure when taking domestic flights -- flying out of BUF, for example, on domestic US flights has had me see this quite a bit. Sometimes the check only involves being asked for citizenship, and other times it may involve being asked for documents.