Like MechanizedWarfare said, you will only provoke defencive-fire from a unit if you move from 1 hex adjacent to that unit, to another hex adjacent to that unit; ‘backing off’ doesn’t provoke (unless of course it has battlefield awareness).
Also note that:
A. if you attempt to move and fail to (eg your humber is adjacent to a Tiger and tries to move into a forest that is also adjacent to the tiger, but fails it’s movement roll) you do not provoke defencive-fire.
B. If a unit is in a smoke screen you can still provoke defencive-fire from it, HOWEVER unless the provoking unit moves into the smoke screen hex, the defencive-fire ‘fizzles’ as the unit doesn’t have LOS.
C. A unit can defencive-fire once per phase not turn For example an AA Gun could potentially defencive-fire three times per turn; once during your opponents movements, air hase, and flight phase. It can also fire normally during your assault. Of course this never happens…