Google Algorithm change?

I just could not understand Google why our keywords which enjoy rank 2 and now rank 17 in just a short while. Just 1 week and we slide to page 2 and 3. The crawled page is there, the back links is there. I check if there are illegal copies from our main site and there are some but these are low PR unlike the original.

I am thinking that maybe there is new updated on google algorithm. Anyone any news or info about Google algorithm changes?