New jobs, new homes, new towns and new endeavors. All can be great and all bring stress.
During transitions we are more vulnerable to affairs. Many new interesting people come into our lives.
As I trained for my first Ironman, I met people from all walks of life. There is an attraction to the commitment training and excitement of being focused on such a big goal. Emotional entanglements can happen easily and then more can easily follow if one isn't aware.
The same is also true of other events.