This is typically due to eating too much starch and junk food. The problem isn't caused by being poor, but rather is correlated with the same bad financial habits
Being poor certainly has something to do with it. The cheapest foods by weight that you can typically find in a grocery are rice, pasta, oatmeal, dry beans, and potatoes. So if you're poor enough that you really have to watch your food budget, you will be eating basically starches with a bit of protein mixed in and maybe a couple of carrots a week. Sure, you're going to be eating less of it, but that may not be enough to offset the sheer carb load.
The case of people ending up penniless after winning the lottery is hardly uncommon, and often referred to as the Sudden Wealth Effect. Pro athletes are also frequently victims of it, and not infrequently lose everything they've earned by the time they're 40 years old. The biggest problem is this: As soon as you're rich, everybody who's ever known you, or kinda known somebody who's known you, or is working for a good cause comes knocking to ask for a handout. Imagine, for instance, that you are sitting on $20 million and your mother comes by and wants your help to buy a nice house: How easy would it be for you to say no? So you say yes to your mother. But now your brother wants the same thing. And your sister. And your cousin. And your best buddy Vinnie from high school. And so on. Former NFL quarterback Bernie Kosar mentioned that a lot of people who knew him had his payday circled on their calendars, because that was their payday.
And for the record, I've experienced both being dirt poor and wealthy enough that people regularly try to hit me up for cash.