My 'rule of thumb' for determining if something's a volley or not depends on the timing of target assignation.
With separate attacks, I can observe the result of the first attack before determining the target for the second.
With Manyshot, I can't - the targets are all determined (and in Manyshot's case, all the same) before I roll any dice. With Scorching Ray, likewise - I can aim all my rays at one, or at multiple opponents, but I decide all targets before I begin. With 3E shuriken, there might be three attack rolls, but again, the target is allocated for all three before the first is thrown.
With Eldritch Chain, I select multiple targets; as with any other spell (or spell-like ability), targets are determined when the spell comes into effect. So before my blast departs, it already has its itinerary programmed in - target A, followed by B, C, and D in order.
For that reason, I'd consider it a volley. If it contained explicit language allowing on to determine the second target after observing the outcome of the attack on the first target, I might change my mind.
-Hyp.