pageoneresults - 4:23 pm on Jan 12, 2011 (gmt 0)
One word - links.
A quick check shows me that they have NOT gained a lot of backlinks this past year - so we can probably rule out that factor. While they didn't increase total backlinks very much, they did gain some new backlinks from domains that never linked to them before, around October 2010. That could be a factor.
Unless they received some very powerful links, I'd rule it out almost completely, it would be at the bottom of my list. Overstock is pretty much beyond the link influence thing, from an external standpoint.
Now, if you were to tell me that they restructured their taxonomy, cleaned up all internal links, are using noindex, nofollow judiciously where applicable, then I would agree wholeheartedly that links are a factor. ;)
Welcome to WebmasterWorld CanadianGuy!