Google's algorithm is changing all the time, even out side of the big named updates, so it's natural for rankings to fluctuate for specific keywords.
Have you noticed any fluctuations among the other sites in SERPs? If that's the case, Google could have rolled out a change that affects queries of that type.
It could also be possible that your competitors have stepped up their SEO efforts over that time, how have their backlinks been growing, and how do their link profiles compare to yours?
Finally, links tend to lose value over time as blog posts are pushed into archives, sites go down or get penalized, pagerank goes down, etc. How has your link building been during that time period? If you're not suffering from an actual penalty like Penguin, the best strategy would be to make sure your on-page factors are solid and continue building better links.