Another thing to at least explore as a possibility: has the subject represented by the keyword seen any substantial media interest recently, particularly in the national press, or on TV?
This happens with some frequency, and can cause big (though often short lived) spikes in impressions. One example that sticks in my mind, from a while ago: keywords regarding a particular women's magazine spiked hugely when Britney Spears appeared on the cover, wearing not a lot. One advertiser that I spoke with at that time had a 1000% increase in impressions that day, followed by a smaller increase over the few days following.
Another example: a small island in the Southern US was featured in several magazine articles recently, accompanied by truly gorgeous pictures and glowing text. Lots of keywords concerned with real estate and rentals on that island spiked all of a sudden.
So, at least scan for national 'news' on the topic of your ads. Sometimes all it takes is one appearance on Oprah. ;)