I have never seen one grow upwards — only sideways or curved side then up. It can be caused by an earlier injury, genetics, or the way a bunny hops (along with I am sure other reasons I am unaware of). A previous bunny, Bailey. who I adopted from a shelter had been severely neglected and she had a crooked spine (due to genetics or neglect was not determined), but she had several nails that grew oddly. Could have been the way she hopped and put pressure on her nails (as she had a odd hop). Don’t know if was diet related or an earlier injury. I just know that in her particular case she had goofy nails. Especially the ones on the out edge.
I also noticed that one of Jack’s nails on one of his feet (a nail closest to the outer edge) started to curl outward. I know that was directly related to how he started standing on his foot as he aged and got arthritis.
In both cases, the nails didn’t seem to bother them, but I did make sure to keep it trimmed so it would have less chance on getting caught on things.