Choosing one’s own food is a basic human dignity that everyone deserves. It’s a primal need and even that choice is taken away from them.
If you’re giving healthy food they might not even know what it is. They may have sensory issues. A lot of people have bought them food and put things in the food to harm them so they don’t trust people. A lot of people can’t eat just anything for whatever reason. It’s usually not personal.
People can be awful. If it’s a woman asking for money and doesn’t want food, she probably has a man forcing her to do that, like a pimp but for money. The food will get her beaten up because she was supposed to bring him money.
If it’s a man then that isn’t usually at play. But if she was legit upset and trying to hide the food, I suspect it’s because she’s being trafficked and going to be in trouble.