Firstly there is not enough shared information.
That said we manage several large sites and there are times were they are static and there are dips, but seasonal factors are huge for most large sites, from education to health insurance. So have you accounted for seasonal factors? Also the traffic growth is exponential at the start and from an outside point of view it seems that the landing page experience is not strong enough or not answering the searchers queries.
I would do a deep dive into onsite behavioral analytics and see how long they are staying on, and where dropping off. In short look for pogo sticking.
Hope that assists.