Cameras design utilizes several different shutter designs. The DSLR (Digital Single Lens Reflex) typically use a focal plane shutter design. Such a shutter is favored when the camera supports interchangeable lenses. This is because the shutter resides at the rear of camera body; it hovers just above the surface of the image sensor. This design features a curtain with a slit. When the shutter button is actuated, the curtain with its slit opening travels across the image sensor. The shutter speed is the clock time it takes to travel the width of the slit. In other words, if the shutter speed is set to 1/125 of second, the curtain moves 1 slit width in 1/125 of a second. If the shutter speed is set faster, the width of the slit is set narrower, if the shutter speed is set slower, the slit width is set wider.
In my opinion, this image was taken during a lightning flash. The flash of light produced by lightening can be quite short (milliseconds). I think the shutter speed was set fast and as fortune happened, the lighting strike caught the lighting flash as the shutter was in motion. The part of sensor uncovered, recorded an image that was well exposed. Part of the slit travel exposed the sensor without the benefit of the lightning strike, this region of the chip is under-exposed. We see this happen all the time when the photographer is using an electronic flash. If the blitz of the flash and the shutter slit motion are not synchronized, we get this effect. Let me add, it’s not easy, perhaps impossible, to synchronized a shutter with a lightening flash.