Going by your information, without seeing it in person ,it would be hard to know the reason the dog does this. Much would depend on the dog's overall temperament. But there are a few aggressions we see in dogs and there are different reasons for this.
I am going to put them below, maybe you will notice something in the descriptions.
I would say though, that it is vital that the dog learn a leave it command and a recall so you do not have to grab him to pull him away and risk getting bit.
Copyright Sally G
There are many different names for aggression, such as leash aggression , barrier aggression, etc. but all eventually are assonated with the dog’s personality or their experience’s in life into one of the aggressions below. Many people also classify their dog as protective, but this too is usually one of the aggressions below.
Fear aggression. Dogs with fear aggression only take to one person and do not trust others , even if the dog has been around them for life. They see things humans do as threats to them. Eye contact, Talking to, reaching out toward or walking toward. If they feel trapped they will act on instinct and may bite. Other animals that get in their space they may go after also.
Fear aggression needs to be treated with care to prevent bites. It is important for people to teach and use commands with a fear aggressive dog so the dog knows what you are going to do as you near it. For instance, if a dog is laying on the couch and you walk up to it to take if off the couch, the dog will bite because it does not know why you are touching it. So you would teach the dog the difference between jumping up on something and jumping off of something. This way you would tell the dog to move off of the couch before you sit. The dog gets off, it knows what you want and it goes somewhere else. Fear aggressive dogs need to be ignored by those it does not trust ,, especially if there are no commands in place. Doing any of the challenges can result in a bite. Positive obedience training such as clicker training should be kept up with through the life of the dog. Behavior adjustment training can also help.
Possessive aggression. Dogs with this will take ownership of items and will growls or snap if someone tries to take them away or they get too close to the possession . They can be possessive over items, food, people, and space.
Possession aggression is pretty much self explanatory where as the dog will not relinquish what it feels possessive of. In this case the dog should be trained daily and put on the nothing in life is free program. All things the dog may feel possessive of should be put away. The dog should be taught to go to a bed or crate in the corner if the room and stay there when company comes, especially children as children do not understand a dogs body language. Training a possessive aggressive dog to give up an item is also a good idea.
Alpha aggression. Dogs with alpha aggression you will most likely see them taking it out on other dogs in the home. They want to be the top dog so they continue to go after other dogs in the home to put them beneath them
Alpha aggression. This should be treated the same as possession aggression. It is up to the owner to be the lead in the home not another dog, so in this case you want to train daily and put the dog on the nothing in life is free program.
Start your training in non distracting areas and gradually move to more distracting areas slowly. If at any point you move to a more distracting area and the dog acts out, then you have to go back to the last spot he was listening and train longer there, before moving forward.
Also, always remember as you start moving to more distracting areas, you need to up the ante of reward . So , for non distracting areas you can use soft dog treats broken up to smaller pieces , but when it is more distracting you need to up that reward to chicken or hot dog slivers.
If you have never used a clicker , you have to load it first before you start training. This just means you do 10 to 15 repetitions of clicking and treating so she knows that after the click sound, she is rewarded. The click sound, as you train, is actually a marker that she has done the right thing. The clicker costs about 2 dollars in a pet store.
Zac George -loading the clicker
Clicker training/positive method training/ print off
For more videos of clicker training other commands go to google search and type in “clicker training videos”
use a variety of items for training leave it so the dog knows it pertains to everything in its sight. Include using a remote control car for moving items
clicker leave it https://www.youtube.com/watch?v=zNAOe1djDyc
clicker maintaining focus http://www.clickertraining.com/files/auto-eye-contact.pdf
Nothing in life is free program
training Give https://www.quickanddirtytips.com/pets/dog-behavior/teach-your-dog-to-give-or-drop-an-item
Behavior Adjustment Training