I've seen multiple possible factors mentioned in excessive hunger on ZC and VLC diets, such as insufficient calories, leptin restistance (or hyperleptinaemia http://www.drsharma.ca/obesity-why-hyperleptinaemia-is-not-leptin-resistance.html), insulin resistance, PCOS, other hormone dysregulation, and low dopamine.
 
Examples of articles that link the above factors to hunger (these may not be the best, as I only spent about 10 minutes searching, you may be able to find better ones):
 
Balancing Leptin - the Hunger Hormone
http://www.stlhdfoundation.org/SLIM_faqs_and_info/Balancing_Leptin_Levels.pdf

Why does insulin resistance cause weight gain?
http://www.pre-diabetes.com/prediabetes/why-does-insulin-resistance-cause-weight-gain.html

The Definitive Guide to Insulin, Blood Sugar & Type 2 Diabetes (and you’ll understand it)
BITE ME, ADA
http://www.marksdailyapple.com/diabetes/

PCOS Symptoms
http://www.free-healthy-diet-plans.com/pcos-symptoms.html

Ghrelin
http://en.wikipedia.org/wiki/Ghrelin
"Ghrelin has emerged as the first identified circulating hunger hormone."

New Food-Addiction Link Found
Mere sight/smell of food spikes levels of brain “pleasure” chemical
http://www.bnl.gov/bnlweb/pubaf/pr/2002/bnlpr052002.htm
[quote]The researchers found that food stimulation in combination with oral methylphenidate produced a significant increase in extracellular dopamine in the dorsal striatum. There was also a correlation between the increase in dopamine triggered by food stimulation and methylphenidate and the changes in self-reports of ‘hunger’ and ‘desire for food.’ “This suggests the dopamine increases during the food/methylphenidate condition reflect the responses to food stimulation and not the isolated effects of methylphenidate,”[/quote]