Hi Jessica,
From my experience, the main reason that email traffic ends up in the "None" bucket is secure browsing (https) with web based email like gmail. Traffic from secure browsing can be identified as direct traffic because it loses the referrer.
Social traffic can end up as direct traffic as well if a url shortened. (like t.co for Twitter) is used.
Another reason might be a cookie wall. If not implemented correctly, visitors that have to "go through" a cookie wall on your website can lose their referral as well.
Hope this helps.
Martijn