Why does my dog run?

I have a 3 year old toy poodle…he is very intelligent and learns new tricks in minutes. He is very obediant until I open the front door. Sometimes when I come home with groceries he sneaks out the door and runs! I’m scared he will get hit by a car even though he returns home.

I even hired a trainer to come to my house every two months to train him with his issue (I also train him alone in between time), but no progress. It’s been almost a year that the trainer and I tried training him not to run the front door.
Any ideas why?