overstock is a huge e-retailer. I shop from them often and love it. 2010 was a big year for them thanks to bing cashback. They have no tax and free shipping on all their products, plus their bing cashback was at around 8% making them very popular on money saving/deal communities. A very popular deal forums I am a member of had daily mentions of overstock so I can see how this played a role in their ranking boost. With all the great deals from them combined with bing cashback they generated quite a buzz and many backlinks which can easily explain their boost in rankings.
It's funny, because I remember early in 2010 I said wow, OS is really going to benefit SEO wise from all the exposure on these deal forums. Once the major players mention something it trickles down to all the smaller communities.