Without know what kind of site you have it is hard to gauge.
Ecommerce Keyword ranking swing wildy as these are often used in conjuction with Adwords and bidding. So whilst you might have a great keyword, if this is a seasonal or on trend product then the competition changes dramatically.
If you have a blog or a company site, not in ecommerce, check what is happening in the industry and see if your keyword has gone on trend. For example: Blue LED. a few days ago this was not the hotest topic around, but since the Noble Prize has just been awarded, this phrase will be more volatile, but will drop back once the excitement drops.
So in essence for the above, if you keyword was rankng well, it might be that you where in a niche area and ranked well even with other SEO issues present. Once competition hots up, others might well outrank you.
If you can give us some more info about your site, that would be a great starting point
Bruce