There's something not quite right but it is difficult to put your finger on what disease it is a symptom of. I'm trying to look not at why my main site has dropped for the most important 2 word term in my niche but rather what has caused the sites that have risen to do so.
Newness, exact keyword domain and rapid backlink building (only in 100 or so directories) seems to do the trick. Perhaps there's something deeper we are missing.
In my niche the affected terms all include a word that has a different semantic meaning at each side of the Atlantic. For other terms and other sites I maintain I'm not affected.