I think most toddlers go through this. IMHO nothing "causes" this at this age. You said he doesn't speak yet, well that's a frustration in itself for all involved. He's expressing himself. Most likely he wants to stay at where ever he is because he's having a good time for instance, so you'll have to figure ways to transition him over to the car. Build it up before you leave to get in the car, sing a silly song about getting in the car seat, or lets see how fast you can get in your seat! Offer rewards for good behavior, and if there is hitting, like the other poster said explain hitting is not nice in a calm voice. Redirect the behavior. Good luck!