Wilburforce - 7:56 pm on May 27, 2013 (gmt 0)
They suggest (not me) it is primarily link oriented.
I personally think - and my own site's behaviour is consistent with but not proof or my only evidence of this - that links are a strong element, but certainly not an exclusive one.
As I have previously posted, a couple of sites in my sector have followed the same pattern for a couple of key terms: several weeks before Penguin slipping from the top of page 1, then wavering between pages 1 and 2, then disappearing (i.e. a fall of 10-50 pages) in the first day or so after an iteration of Penguin. Three ex #1 sites inlcuding mine have all gone the same way.
As I have also posted (make of it what you will), for several weeks three ex-page 1 sites for my main key term were all on the same results page (page 48, from memory) a month or two ago.
Somewhere in all this I suspect there is a poisonous Panda/Penguin cocktail: if some on-page issues are flagged AND linking issues are added, down to the bottom you go.