Physical Danger
The life of a nuclear materials courier can go from zero to a hundred, real quick. Most of the time, it'll be at zero, where the courier is making uneventful pick-ups and deliveries three weeks out of a month. Can you say back and knee problems? On top of that, while everyone's sound asleep in their beds at night, couriers are out in the middle of the road carrying around stuff that could blow up three states.
Keep in mind that couriers don't go through extensive gun training for nothing. They need to be prepared for situations in which they're under attack and must defend the nuclear materials. Nuclear materials are extremely dangerous; imagine the damage if someone was able to steal the materials off your truck. Pretty bad, right? And that's why couriers are prepared to use lethal force. Things could go from zero to one hundred pretty quickly in this line of work.