I'm sure this depends both on the person and that bad habits in question. But I can see how this happens, I have seen several people on a smoking/eating cycle.

They try to loose weight, go on a diet and end up smoking more to keep their mind off food. So then they try to kicking smoking, and starting eating more to keep their mind of smoking. And cycle back and forth. As long as they are trying to kick only one bad habit they don't get anywhere.

Jay