One of the most important feature that was introduced with Google Panda update was penalizing websites with duplicate content. A lot of highly authoritative sites started to get penalized by this update and one of the main reason turned out to be the duplicate content issues. Some website with authoritative links to many .gov websites also took hit and up on careful review these websites in spite of their authority had unbelievably large amount of duplicate content in them.
With the Panda update Google is redefining page authority based on quite a few standards, but by the few reports learnt from Internet as to why many authoritative websites took a hit, it became clear one important reason was the duplicate content issue. These websites on removing the duplicate content started to rank better in the search engines.
One more factor that Google has been considering is domain age. A domain that several years older seems to have more credibility in the eyes of Google search engine rankings. An old domain necessarily does not mean an automatic top ranking in the search engines and it needs basic on page techniques too but it definitely has an advantage over newer domains. Newer domains which quickly shoot up to prominence seem to be more under scrutiny with that Panda updates thus controlling the effect of black hat techniques that might try to demote a much more authoritative and older domain.
One more reason why an authoritative website can get demoted could be because of the duplicate content issue from other websites that has copied the content from the original site. Some do it on purpose so as to bring down a competitor rankings. Identifying and requesting them to take down the content helps in reviving the rankings again. Some go through the process of lawsuits which drag the process much longer.
