There is another thread about a terrorist threat for a tunnel from Detroit to Canada today, so possibly they are checking for potential related activities.
That was my first thought as well...sounds a bit like the traffic stops around Atlanta a few months ago. As I recall, every truck on I-20 was being stopped and searched (during rush hour), and more than a few were being scanned by some sort of radiation detector. ICE was out there, along with the FBI and state and local law enforcement. Not to worry, we were told. It was just a “training exercise.” Expect a similar explanation for the stops in NH.