Originally Posted by MSPeconomist

I would guess that the crew in customs is deadheading on your flight in order to work a flight tomorrow. I wouldn't expect a crew arriving from working an international flight would be able to immediately work a domestic flight without a rest period. Alternatively, the crew could have been deadheading on the international flight before working your flight, but that seems less likely due to rest rules and the implicit cost of having a crew deadhead from an international airport rather than working an international flight from their location.