This isn't something that "just happens." By any chance did you skip a recent farrier visit (or reschedule it)? We rescheduled ours once and two of our horses began to flare already, even though they weren't long (a couple weeks) overdue for their trims.
That looks like a surface crack that one of our Appys has (he's a porker). Cracks like that, our farrier says, is in the hoof wall (just the surface; he wasn't concerned about it and said we shouldn't be) is under stress from weight and gets "stress cracks."
Is your horse heavy on the 3 flaring legs (underlying lameness issue in the not-so-flared leg, causing the horse to put more weight on the other 3)? We have two horses that are "front heavy," and because of this they are much more apt to flaring and being "flat-footed." It's something that a good farrier should be able to fix.
However, be prepared for it to take a little while. Like with anything involving horses, hooves are not something that can be rushed along.
"Adapt or perish, now as ever, is nature's inexorable imperative." (H.G. Wells)
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