Your content being copied has asloultelly nothing to do with your drop in ranking.
The usual reason for drop in ranking is simple: other websites got more/better links than yours.
We are being copied on a daily basis and I am sure many others do too. There are services which specialize in content carbon-copy, such as press release agencies (the same press release slapped across hundreds of sites) or you can see the AP model – the same news story with the exact same wording on thousands of newspaper websites throughout the country/world without any issues whatsoever.
Content theft is indeed a nuisance, but when your website goes up or down the rankings, it’s usually the fault of your own actions.