I contend there are only two ways to go back. Drinking to loss of reasoning or talking themselves into it.
Those are how it happens but what about why it happens?
I believe the main reason is because we started so young and it's all we knew.
It is uncomfortable to let go of something so ingrained in our memory banks and lives.;
The other reason may be they never were able to let it go and they fought quitting the whole time.
Of these two, I believe that holding on is the most dangerous.
The solution to that one is being willing as you quit instead of reinforcing the "missing something" by dwelling on that aspect frequently.
This is why we tell you to start doing new things. Things you might not have done before to show yourself change is in store. Positive things. Things that give you a mental reward.
Quitting and succeeding at it forever is all in your state of mind.