I figured out one of my dogs triggers. How can I curb/deter it?

So I've learned one of my pup's triggers.

It seems whenever strangers reach their hand over or near/above my dog and get close to me (like trying to hand me a card/bag or something), he becomes extremely reactive and aggressive. He will lunge and try to bite the person doing it.

Is this considered a territorial thing?

He can meet the person, like them, wag his tail, and let the person pet him, and be okay. But as soon as they reach over him while he's next to me (ex. A man wanted to give me a business card), my dog instantly gets aggressive and lunges at the person, moreso males than females.

Any tips on how to curb or correct this? Idk if it's fear, territorial or what.

I also posted with the reactive dog community. But appreciate all insight I can get.