Okay, so you can put Panda aside because it appears to be a non-factor.
What you have to do now is instead of saying that "the overall trajectory is downward" is get under the hood and get your hands a bit dirty by trying to identify the *exact* hows and whys of the traffic drop.
Was it certain key phrases? Certain site sections or pages? How has overall search volume changed? What changes have there been in the competitive landscape? Has the move to mobile affected traffic?
There are many other questions that might be asked. Start by getting answers to some and you might get pointed in the right direction.