Is there a reason he might be worried about the door opening? Did he have a bad experience outside? Maybe it's just one of his things.....all cats are different and some are stimulated by WEIRD things!
Sugar loves to greet me at the door and Twinkie runs around like someone is after him. After he knows for sure it is just me he comes out slowly and cautiously hehe, he is such a chicken. Anyhoo when someone bangs on the door or knocks both cats freak out and Sugar starts growling and Twinkie is nowhere to be seen. I think your cat is just a bit scaredy cattish, some cats are just like that, as long as he likes being petted later, he is just a bit paranoid like my Twinkie