A prospective buyer asked; are the bags I make and sell stiffened with anything? How do they stand up like that? You know, I never really thought about it. They DO stand up don't they?!
I'm guessing it's the double layering on the inside and bottom that does it. Just to make sure they stand up, I took a skein (my trekking yarn skein) and stuck the project in a bag at my feet. 20 minutes later, it's still standing up. I never noticed that, but they do!
Now since I don't add any stiffener to them, they will still stand up by themselves after being washed. In a stiff wind, they do fall down. There were a few of the earlier bags made with thin fabric that won't stand up for long. I started backing them with fusible interfacing to make them stiffer.