Every once in a while, though we don't really know how frequent this is, big G updates its search algorithm. Most changes are not really felt much, but huge and significant updates, like the one released over a year ago, coined as the big G's "Panda," ravaged through spam sites and content farms alike. HP was affected, but as you see we've recovered well.
That doesn't seem to be the case however before the end of February 2012, when the Panda was again updated. It seemed that many content sites, including ours at HP were hit significantly. I cannot say anything about how the Panda devalued our hubs because the algorithm is a best kept secret. What I can say however, is that you should do some link-building once more and in due time you'll see your rankings high up once again.
If you would ask if I'm affected, well, my traffic was 75% lower this month compared to before the update. You're not alone my friend.