It stems from when I was still a little filly in Equestria. They say it must've been INJURED from when I was small. Like, uh, like maybe I fell wrong as a tiny filly and hurt it just enough to crack it.
So, um, the wing healed the wrong way, and I haven't been able to fly HIGHER than, like, FIVE feet before I fall.
So they have to re-break it, put in a few pins, and reset thing so it heals right this time!