Shaddows - 3:23 pm on Aug 29, 2012 (gmt 0)
Don't forget the other half of what I said. Google needed time to profile your site, and to confirm what query intent your site best served.
Another possible explanation along the same lines it that you have "graduated" from Long Tail obscurity to Fat Middle yuppiness.
In any case, your site is too young and its profile too immature to read very much into vagaries of referral shifts. Google is likely to profile and reprofile you as your place on the link graph becomes more defined, and semantic signals ramp up over time.
If you seek opinions on how long it takes before a site behaves "normally" in SERPs, you will get a variety of answers, but I've never seen anyone suggesting its less then 3 months. My impression was that 1 year seems to be a consensus opinion, but
a) that impression may be wrong (its not something I've been actively monitoring)