New pups food Behavior

[Naldo]

Hello everyone, this is my first post. I have a question about my new rott Bosco that I sort of adopted about a month ago, I am guessing his age between 2-3 months old going by his height (15cm).
My question is about aggression he has towards his food. It's kind of weird because his aggression is only towards his non-normal diet. He gets Canidae dry as his main source of food along with wet food, and home cooked meals (meats, vegetables etc.) every once in a while, and his normal treats for training.
With his normal diet he is comfortable with people around his food, maybe a slight growl sometimes(very rare), but nothing major. The other day I was eating a sandwich from subway and I left the plate on the table for a minute and he knocked the plate over and started to completely maul that sub. When I told him no and tried taking it away, it was like he was a completely different dog. He snapped back gave me a good 2 inch cut on my hand then he gave me multiple "about to attack" type barks which I’ve never heard from him before. As soon as I took 2 steps back he kept eating. I quickly grabbed him by his muzzle said no and put him in his crate for a while.
Any Ideas as to why he gets like this, and only towards foods that are not part of his normal diet.

[zorro13]

Why? It's a subway sandwich, it's a high value item, and you left it on the table for him to access it. Now would be a good time to teach your pup about food being left alone and/or dropped on the floor and to not touch it/eat it/nor think about eating it. You start with a treat, holding it in front of the dog, telling them to wait or no, then giving a word, like 'ok' to then take the treat. You then build up to putting the treat on the floor in front of the dog, then build your distance away from the dog when you see him maintain his 'hold' position until getting the release to eat the treat etc.