Why lightning travels in zig-zag is easily explained. It's not an unexplained phenomenon.
The air is normally an electrical insulator. However, for lightning to happen it must breakdown so that a conduction channel forms for the flash to happen. Also remember there are water and dust particles in the air. So the breakdown does not happen all at once. It happens in discrete steps approx 50m or so with each step taking 2 microsec to 50 microsecs. Basically jumps from point to point with the least resistance path and hence zig zag nature.