I've been able to crack my neck for the last 6 months (maybe even more, quite hard to remember when I started it).
I haven't mentioned it but the cracks aren't involuntarily, I've just became so addicted to it that it is really hard to stop with it.
And maybe it is a product of working too hard, meanwhile I stoped working in gym, just in order to isulate potential causes, but I don't think that this is the cause.
How about if you try to go a day without cracking? Do you just get a strong urge to do it that you can't stop yourself from doing it?