Even people who (correctly) start with opposite hand/foot forward . . . will still tend to take that step back if the rear foot is not properly grounded.
There should be firm pressure of *both* feet in the set position. Then forward propulsion will begin with a brief push of the rear foot and then a more noticeable push off the front foot.
In a sprint start, the action of the legs begins with a brief but surprisingly strong push on the rear block, then the push that everyone notices from the front block. This is why coaches emphasize loading *both* feet firmly into the blocks. The same principle applies to standing starts.
In my experience it takes about ten minutes to get people to understand the principles of a good start, and not much longer for it to feel "natural." Coaching--conveying the value of not giving away a tenth-second or more at the start--will help.