Boundary guarding. He has a boundary (the door) and he plans to protect it.
Mine tried to do this. Wanted to shove in front and give everyone a sniff down before they can enter. Didn't escalate any further because I stopped it when I saw it, but that was the start of it.
He has a "place command" where he goes to a mat when I ask him. I taught him that if someone comes into the room, he needs to either not move or I will "place" him if he gets up. He is not allowed to greet people at the door. He has to wait for them to enter the room before he can be invited to greet. Basically what needs to happen is you have to "own" that space (the doorway) in your dog's eyes so that he doesn't feel the need to guard it. I would not let him approach the doorway without permission when someone is coming through. This is also a good time to use a down stay if you don't have a place command.
With any guarding behavior, it usually boils down to a lack of ownership on the human's part. Dogs won't guard things they know that humans "own". My dog loves playing with his ball and his chew stick, but when it comes down to it, I've taught him that I own those and he will give them up readily to me. Same with the door. When people come through the door, I own that entryway space. When I am ready, I will invite Kaiju into it to greet, but he does not control it, therefore there is no need for him to guard it. Always remember that just like food, toys, and chews, dogs will also claim and guard space if they believe it is important. It can be as simple as a comfortable corner of the room or a doorway. But if they feel that they can get away with owning space like that, especially space that regularly has humans passing through it, it can definitely be a problem.