>I don't see this as a lightning strike at all.
>It looks like it was sourced from inside…
Building wasn't properly grounded. (My best guess)
The building was under construction, for starters.
The higher up one gets into the atmosphere, the greater the voltage potential there is between one and the ground.
But really though, the storm likely played a huge role here, as that too would bring extra charge into the equation.
The building itself might have been collecting free charge from the atmosphere, building a capacitance, until the voltage reached a high enough potential to bridge the arc gap.
Just throwing it out there. I'm not really an expert on anything.