Firstly if you haven't been updating that is why. Search Engines like active websites with updated content. Especially on blogs.
However sometimes sites go off the scale for a phrase when they are about to move position for example I had a site rank #14 drop out to around #200 and then come back a few days later at #6 I don't know why it does it but it only seems to be google and it normally only happens with newer sites.
However by guess would be it is due to not updating your site while your competitors for the same term have been doing.
Anyway hope your rankings improve again
