I'd say it's his age, he's pushing boundaries trying to get away with dictating when you leave the field which to him is the best place on earth with all the scent to follow etc.
Plenty of recall practice in that field clipping him on then letting him back off after a few minutes and a nice high value treat (pig ear or similar, something that will last a few mins) whenever you put him in the car after a walk will have him sorted I reckon.