I've one plant that has the curled, or 'hooked' leaves.

It's growing alongside another plant that hasn't shown any signs of curling, in the same container at that. That is...both plants are growing in identical conditions.

So I did up some clones, sure as shit, the clones of the curled mother plant right away began showing the same effect. A node starts out healthy enough, but as the days pass, it hooks over when a new node begins to develop. With all things being equal, the results led me to believe, at least in the case of the one plant and her clones...

It's genetic.

So, if you've any cloning to you...do up a pair of cutlings and see if the same effect continues on to the clones.