Whilst there could be a million reasons.

I think a lot of people don't realise or perhaps consider that moving from school/University to the workplace can drastically change the amount of time you are moving throughout the day and your physical activity.

I used to work in a retail environment where I was on my feet all day in my early twenties, then I moved into an office environment. During both workplaces I exercised a lot but I absolutely noticed that I needed to be more conscious of what I am eating after having started working in an office.