Who moved my traffic?

Your site traffic is down, you are running up and down the hallway freaking out. Who moved my traffic, who moved my traffic. Wait before you get all anxious about the traffic. Downturn in your traffic might not be something to freak out.
Below are some of the things to look at to find out why your traffic is down. Some reasons are within you control (stop freaking start working) some out of your control. Some might be very obvious and some might not.

• Seasonal Impact - Do year over year comparison and see how the traffic pattern was last year.
• May be overall traffic is down even for your competitors (do comparison at http://www.alexaholic.com/ )
• Has a new competitor entered the space? How is their traffic?
• Traffic drivers - How was the traffic from these sources?
  Campaigns (Banners, Search, Emails) - Did they ran as normal.
  Email - Did anything change there? Did you send out your regular emails, newsletters?
  Search - Did you change anything here, has search engines changed their algorithm.
  Search - Did you change your site? Meta Tags? Content?
  Affiliates - Has any affiliates changed their site.
• Environmental Factors - How is the weather in the geographical region where you have most visitors from? Nice weather can keep people outdoors, resulting in lower traffic.
• Was there any site outage?
• Have you made change to your web analytics tool configuration? If yes, investigate what those were? Problems in filter could be filtering out a lot of traffic.
  

This is a work in progress. Feel free to send me other things that you might look for.