tedster - 11:27 pm on Feb 12, 2013 (gmt 0)
This can be a very touchy analysis. It depends on how far your URLs fell on various keyword searches.
1) If you dropped 10 or more positions, then something in the algorithm probably decided to demote your site. Looking for that factor is one kind of analysis. It could be either a recent change that you made, or it could be a change in the algorithm that Google made.
2) If just one of your big keywords dropped just a few positions (say from #1 to ##4 or 5) then it may be about a different kind of algorithm change - one that is rewarding your competitors' sites for some factor rather than demoting your site.
The second type can be very difficult to analyze if you haven't been keeping a detailed record of your competition. However, an eyes-open inspection of the sites that moved up may show you what they are doing well on that you are not. Don;t think about only technical factors in this inspection, think about how happy the site is making its visitors.