You could have tripped an over optimization filter or some of your backlinks may have become devalued.
ranking fluctuations are normal, especially is you have a newer site....the more links you gain, the more your site usually stabilize
The more quality links you have the more shit you can get away with Too many links may "de-stabilize" your sites ranking
one of my site going 2nd position to out of cache.This happened on today morning.plz any one suggest what i need to do.