Answer:
The word success implies completion. Completion of a goal, or project which usually affects the persons life by improving their state of existence. A person becoming a lawyer can be seen as beinf successful because they completed the requisite trials required to become a good lawyer. Their earning power, knowledge, and opportunities have increased.
So, once a person is successful they are in a new state in their lives and that brings new challenges, problems, circumstances, and even new goals. Therefore, if the person is unable to navigate these new dillemas, they can fail. That failure might effect much more since they were at a point where everything was getting better.
Combine this with the fact that all of us fail at some point and that is how I think success can lead to failure.