I have no idea either. Our dogs are actually quieter in the car than at the house. Jax barks when anybody comes to the house, but if someone approaches the car, he doesn't bark.
Do you get your dogs riled up before they get in the car? We always tease Jax and tell him we're going for a ride probably 10 minutes before we actually go somewhere - then he's all excited. If you do that, you might want to stop because maybe that makes them hyper. But if you don't do that, then I'm not sure what it could be.