"Off" and "away" are prepositions that have different meanings. "Away" refers to increasing distance. For example, "the car backed away from the garage". "Off" is the opposite of "on". "The cat is on the table" and "the cat is off the table" have opposite meanings. You cannot say "the car backed off the garage unless it somehow drove off the roof". If someone is behaving aggressively towards you, "back off" works better than "back away" unless they are being physically aggressive and want them to physically move away from you.