You! Yes, you! This medicine needed to be delivered YESTERDAY to CSW, ya'hear?! They willin' to pay a premium for it they need it so bad, but ya gotta be quick! It's not as useful if its late, ya'know what I mean?
A more serious explanation: rarely, a high priority freight haul job will spawn that will have a significantly higher time bonus pay, but with a more restrictive deadline (maybe ~25% shorter?). Maybe a city is in desperate need of medicine, or the food factory needs enough produce to meet the holiday season deadline, or an emergency order for ammunition has been issued to MB. Whatever the reason, the goals are the same; get it there FAST.
High speed, high risk, and high reward is the name of the game! (RIP that poor squirrel and your copay)