A USA Today article cites that negative SEO might have been a factor in Expedia's ranking drop.
Expedia may have been hit by a "negative SEO" campaign that hammered the travel website's rankings on Google searches, according to an analysis by the firm that uncovered the problems.
I can't imagine that negative SEO would have such a serious impact on a large internet brand like Expedia. Sites as large as Expedia to be hit by negative SEO is almost impossible. Unless the whole thing was staged and it fooled Google, which is possible, but unlikely, I can't imagine that this would be negative SEO.
(1) It would make Google look ridiculous.
(2) It can set Google up for legal worries because it impacted their shareholders.
(3) It would make it possible for any huge site and publicly traded company to be gamed both in search and in the stock market and that is way too scary.
Robert at WebmasterWorld added a funny point:
Worth noting that the links from the USA Today online article are apparently not nofollowed. Maybe this is part of the Expedia recovery effort.
Forum discussion at WebmasterWorld.