Looks to me like it was trying to root from the leaf. Lots of plants will do that. Begonias for example root from leaf sections easily.
I however have not had a hoya really vine well and actually grow from a leaf rooting. A wound in the leaf, such as seen in the bottom part of the photo will often root. It's a defensive mechanism.
When a leaf fell off one of my plants, I planted it into the same pot, but then I threw the leaf away when I read that even though hoya leaves can root from the petiole, vibrant plants almost never result. I'd still love to see what becomes of your weird rooted leaf.
That IS an oddity, Carol! I've never seen one root on an edge, either. I've rooted many leaves over the years - I was told if you get even just a bit of the meristem, it will grow, but that rarely happens when a leaf gets knocked off. I still root them back in to the original pot figuring if nothing else, they serve as a bit of "ground cover".