Thought about this but it's better for them to be behind, because the mechanic can simply jump out of the car and push the bike right to the cyclist in trouble with perhaps the car even moving slightly forward.
If ahead of them, the car would have to stop, the mechanic get the new bike and have to either carry it to flip it in the wrong directon to the cyclist in trouble, then have to flip it again in the right directon taking up a perhaps a few seconds of time.
Also the car ahead of them just can't stop and go backwards to help them, but being behind, they can pull right up to the cyclist in need. And being behind, the car can see them in trouble but ahead they would be completely reliant on radio communications.