This may have started out as a fear response but it seems to have developed into a 'knee jerk' reaction when ever he needs to make a point. I have a feeling there's aren't many warning behaviors in advance, he has become reactive and he needs to learn to become thoughtful. He needs to learn to think about his behavior and check in with the people for guidance not just get nasty.
He learned that 'snapping/biting' got people to back off and he learned it works, and when a dog learns something works they do it more. It becomes self rewarding.
It sounds like he rules the roost. Some good basic obedience could go a long way with him but he could also use desensitization exercises and boundary setting. He needs to learn that people are not a threat, people set the rules and boundaries (not him) and protecting his things will only get him further from the very things he is trying to protect. For example if I went to pick up a dog toy on the ground and he lunged for me to protect it then my training partner (who has him on the leash) would simply walk him away from the toy with a 'leave it' command. Then we would repeat this drill until he learned that the toy is not his to protect and bad manners will not win. Being calm and patient wins every time, not a
One of the biggest mistakes people make with their dogs is they give them way too much freedom - we call it 'recess'. Dogs need to be in school even at home. They need to have their minds engaged, be taught rules of the household and learn respect for their family members. This will fix most any problem you would possibly encounter with a dog. We can't just let them roam about the house 24/7 thinking "what do I want to do now? Go get a drink, chew a toy, bark at a squirrel, get a pat from mom". Because it becomes all about them and not about you. They become impatient, independent, impulsive and reactive - this is where ll of the bad behaviors show up. If you engage their minds they become connected and thoughtful, and the impulse becomes "what does my person want?"
Love Them & Lead Them,
~Elizabeth & Doug
Dog Training the Way Nature Intended