Don't know the state of current research is.  This is topic I sloughed-off due to lack of time a long time ago.  The only book I can find in my library, at the moment, is Gimpel's The Medieval Machine written for a 'high brow,' not academic, audience and it's over 30 years old.    

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre

And the black plague.  That didn't help, either, though its role was more to finish off a declining system, rather than to start the system's decline.

Too many piggies for the teats.

Can't remember their ag complex yield but it was, by today's standards, pitiful.  You can start your research href="http://www.cropyields.ac.uk/project.php">here, if you want ;-).  They would get, roughly 4 bushels for every bushel of seed compared with, roughly, the 20 to 30 bushels modern farmers can expect.  This meant they had to increase the total number of farms by using ever marginal land in order to try and maintain food production.  Eventually, they farmed areas that are not in use for field cropping even today.

Then the weather turned on them, the marginal land went out of production, some kind souls invented the Black Death, and it was all over.  The European population didn't recover until the introduction of cultivars from the Americas, particularly the potato.


She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre