I think you answered your own question. They got off track. Our bodies are programmed to stay at a certain weight so any lack of life-time vigilance is going to be rewarded in this way.

Its really frightening how few people keep it off. Something like 5%. I think a lot of it is because most people go on diets and think of it as a temporary change. If they don't continue the diet their entire life, they'll gain back the weight. That's why people are saying "Way of Life" that know they have to keep it up for the rest of their lives.

Fat cells have long, long memories!