The dog was protecting you but at the same time he was probably scared and thought that your Mum was shouting at him. Most dogs understand who is in charge and will be submissive to them but if push comes to shove and he thinks another "pack member" is in danger some dogs will stand their ground.
One of our dogs hate it when we play fight with the kids and will always try to get in the middle and stop us. He's never gone as far as growling or biting but he does grab hold of your arm sometimes as if he thinks you are really going to hurt them! It's good to know that he would protect them if they needed it.
