So what happened to these five people? The following video explains it by using Jessica Simpson as an example:
Essentially: Before weight gain a person is paying attention to calories and exercising frequently/regularly. For one reason or another, either the calories increase or the exercise decreases or both. Once it starts, the obesity death spiral can take hold, where increasing weight makes it harder to exercise, so exercise goes down even more, and paradoxically calories may increase as self-control goes out the window.
Speaking from experience, it would be great if you could never let it happen. But if it does happen, it is reversible: