Maybe ISP pulled them over just to get a better look at the inside of the car and its occupants.
Another possibility - getting him stirred up and worried to prompt him to ditch the weapon or any other evidence he might have had with him. I'm guessing LE (FBI/Idaho SP) was following the car carefully, most likely with multiple vehicles. They probably had a plan in the event he threw something out of the car, for one car to stop and look while the others continued to trail him.
In the recent trial for George Wagner IV for the murder of the Rhoden family, it was revealed the police planted a mic device in George and his brother's big rig. They then texted a photo of one of the murder weapons to George's brother. They both hit the panic button and went on a yelling rampage that lasted over an hour, all recorded on the recording device.