A lot of times, once a shoe has come off prematurely, the wall is torn or otherwise compromised just enough it's harder to place the nails, as there isn't much growth as normal. Unfortunately, this makes it even easier for the shoe to come off again. Even worse, is that each subsequent time it falls off, there's less for the farrier to work with. Horses with thin walls have the problem the most (horses that people are told MUST have shoes, because they are so thin walled, ironically). As the wall keeps getting more nail holes and chunks ripped off, it's more prone to infections and soreness.
Sometimes it's just because a horse likes to paw with that foot, or has figured out how to pop them off by hooking the heels over a strand of fence wire/board and catch the heels. I've had people call and tell me they HAD to go barefoot cause the horse would promptly walk over to the fence and pop the shoes off. It was too expensive to keep putting them on.
Ask the farrier to make sure to use the smallest possible nails when replacing the shoe and make extra sure that the heels don't extend too far and get caught by the hind foot. Also, making sure the hinds are trimmed so that they don't overreach could help. Glue on shoes are available. You could let her go barefoot and ride with boots for a while to regrow the hoof enough to have a secure section to nail to.
But of course...me being me, I'd say just forget the shoes,
if she keeps popping it off it's doing much more damage than going barefoot intentionally would ever do. Horses that "HAVE to have shoes" usually suprise the owners and switch over to barefoot quicker than some of the more average horses, and never miss a beat.