Bing mystery solved: After the 2nd or 3rd time this happened with a site of mine (a major rush of traffic for one particular keyword phrase), I checked msn.com and found bing search results linked to on the page somewhere. Sometimes it's a block on the page with different searches for the day, other times it may be one or two featured more prominently somewhere on the page (especially for hot seasonal queries).
A page from my site would be in the top 10 on Bing (on high visitor days). The stampede of traffic is no joke, lots of browser home pages must load msn.com. It's definitely a nice perk from Bing.