If you feel it is fearfulness or aggression, it's important first off to try and avoid any situation it's occurring in for now until you can get with a behaviorist for a consultation. There are also a lot of excellent books out there on positive training methods and helping a dog to understand his/her place in the pack (without use of force). A lot of times simply our behavior and body language can reinforce negative behaviors without our even knowing it. It is of utmost importance to get this addressed now before it gets any further, especially if it's not done in playfulness but is done in fear or aggression. Also anytime a person jumps back from him, the behavior will be reinforced.