One of the reasons for a traffic drop can also be due to your site losing links. You may be seeing a direct loss of that referral traffic, but there could also be indirect effects. When your site loses inbound links, it tells Google that your site isn't as authoritative anymore, which leads to lower search rankings that in turn lead to traffic drops (because fewer people are finding your site if it's not ranked as highly and more).
One of the things that can slow-bleed the traffic from your site is the quality (or lack thereof) of the content you publish on your site. Previous Google updates like Panda have already been released specifically to deal with the issue of low-quality content on websites. Long story short: Panda intended to stop sites with bad content from appearing in the search results.
Beyond organic and direct traffic, you must understand the difference between all of your traffic sources and how traffic is classified. Most web analytics platforms, like Google Analytics, utilize an algorithm and flow chart based on the referring website or parameters set within the URL that determine the source of traffic. Here is a breakdown of all sources: