I think careers go south for the following reasons

1. Fighter is given low quality opposition and then moved up to quality opponents too fast.

2. Management sets them in with a bad style opponents for their style.

3. The don't fight often enough and get ring rust.

4. They get old.

5. Can't handle the pressure of being on TV.

6. They lose the fire, to keep boxing.

7. They start thinking about money to much and not enough about boxing.

8. Go back to life of thuggery

9. Start partying all the time getting drunk and using drugs (it's ok for the fans, but not professional athletes)