It's genetic in my opinion. I had a cross bird that developed it and hatched some of her eggs - a very small number, as I recall (she was an "olive egger"), and one of her female offspring developed it as well. I feel like it results from crop statis (crop not "moving" food along as it should). I also had a bird of another breed that developed an extremely pendulous crop; I removed her from my breeding pen & put her in the layer pen, where she lived without any health issues to be about 5 before passing. I sure don't recommend keeping them (I was new to chickens back then and didn't know any differently), as they do suffer since they're not getting the nutrition they need. Since the crop moves too slowly, they're not getting nutrients, which compels them to eat more, causing the pendulous crop. The food can go rancid in the crop.