Possibly a combination of both?
It doesn't look like they've been zapped by high-voltage; there wouldn't be much bird left if they had. It would be consistent with a loud bang, though. The insulator spacing on those high-voltage lines is significantly larger than a starling.
However, it seems likely that they were sitting on the power lines - as birds are inclined to do for prolonged periods and in large numbers - when whatever killed them happened. Gas from the LNG plant seems plausible. Maybe carbon monoxide (or something nastier?) from a flare stack or something? Or perhaps ionising the air sufficiently to bridge the circuit and electrocute them, without fully arcing over? Or they were poisoned, and happened to be congregating on the power lines when it took effect?
I'm reminded of a (probably apocryphal) story of microwaving seagulls in-flight with military targeting radar...