I don't know what causes it... I've heard all different kinds of stress can do it. I've had it happen a couple of times to clones that went into smallish pots and had to wait quite a while, sort of slow-vegging (not the greatest light, small pots) until I needed them. Once when I threw one of those freaks in with some healthier plants to flower, the result was miserable - not even worth the space it took up. Another time, I re-potted the mutant into a bigger pot, gave it some better light (still vegging), and it went back to normal. It did take it a while to get back to normal though (but, it did turn into a good, healthy plant).
I guess that info probably doesn't help a whole lot. Probably about all you can take from my post is that you ought to get them back to normal before trying to flower. At least that's my experience.
Hopefully someone else will have more info. I'd be interested in knowing more about it myself.