What I have noticed with my wife:
My wife didn't work for many many years while the kids were young and growing up. Now that they are teenagers she decided to work part time. When she is working it has her barely going to the gym to workout. Not to mention she tends to eat a little worse for lunch and snack a bit more. Followed by coming home and putting her feet up watching TV and eating a bag of chips.
With my wife when she is stressed either from work or whatever she tends to gain weight. Also, I have noticed the best thing for her to really gain weight is to build up her appetite. Once that happens the flood gates open....even if she works out she eats so much more that she still gains weight.
As most have said being sedentary. Which my wife gets after working.
Also, eliminating exercise and workouts. But with my wife she sometimes needs that in order not to diet or to feel good about herself....and if her appetite increases she still gains while working out a couple times a week. And when she gets pretty heavy her workouts fade away or become 20 minutes vs. her normal hour usually followed by an extra huge meal and snacks.
8 years