This is a behaviour I've been observing for a while, well before Panda, where pages optimised for a particular keyphrase simply don't rank but other pages very loosely related to the keyphrase rank instead (and not very well).
While the rest of the site behaves as expected (similar level of on-page optimisation, similar internal linking, similar quantity and quality of content, similar level of keyword competition) and most pages rank consistently in the top 10 for their main keyphrase, a small number of pages simply behave abnormally.
The best way I can describe it is that it's almost as if Google is refusing to rank THOSE pages and ranks other far less relevant pages on my site instead. It's as if something about these pages offends Google and I don't know what as they don't really stand out when compared to the rest of the site.
Has anyone observed this? Any ideas what to look at? How do you determine WHAT Google might find offensive about a particular page?