There are a few reasons why your dog may show signs of travel anxiety. One of the main causes can be motion sickness. This typically happens because a puppy is still developing and regulating their ability to balance. Your puppy may grow out of this but it is possible that they start to associate car travel with the feeling of being sick. For more tips on how to combat this, please check out the below graphic.
Graphic created by Ancillary K9, a provider of one on one dog obedience training.