It is hard to tell what is going on if you are only going by traffic. Perhaps one site does good during the holiday season and another does not. Maybe there was a world event and all of a sudden no one is interested in your site. There are a lot of reasons why one site will fall for a week or two.
I usually do a quick check in Analytics and see how many pages were seen. And then I compare it to similar days (like Fridays to other Fridays etc). Was it just a bad Friday or Tuesday?
If there is a trend, then I check my SERPS and make sure they are still showing up in the same spot they were before. If the SERPS are showing up, then it was just a bad day or week.
It is difficult to see what is happening if we just look at traffic. Why was the traffic lower? What happened to the SSRPS? Was there other reasons outside our control?