I have had this happen a few times when coming from overseas.
One odd behavior I have notices lately: On my regular (now cancelled) flight from AUS-YYZ, the US CBP had officers in the jetway on the way to the Air Canada plane and they were checking passports and asking what I would call "extensive" questions. (people on work visas were asked a lot of detail what they were doing, people on leisure were asked day by day or week by week what they had been doing, a minimum of a few minutes for each passenger).
It seems to me like it is probably a case of too much CBP manpower in Austin and not enough to do?