I think it's mainly because she likes to crib while she eats.. however, in her stall, she doesn't have to worry about the other mares. Outside, she's usually lucky to be permitted near the hay. She really is at the bottom of the totem pole, so she's often told off. When she's inside her stall, we usually put it to the farthest corner so that it wouldn't be as convenient for her to walk over to her stall gate and crib.
However, with the hay feeder.. it's right there for her to crib on, whether she's eating the hay or not. Apparently all she does is crib all day on it, and never really ventures off.
Forever loved, never forgotten; my beautiful Indie. <3 Hoofprints on my heart.