$\begingroup$

I won't repeat something provided in other answers. The only thing i would like to add is that we don't see a Ground To Cloud Lightning.

Theoretically speaking Ground To Cloud could be possible since lightning is a discharge between 2 points that have extreme voltage difference. So it is not a mystery "why this happens" as said in this video, but 'how this happens".

Is it a case of Geology of the location (like Lake Maracaibo or the lake of the thousand lightnings)? Yes and it is needed to examine more thoroughly since some effects do happen more frequently in certain geographical locations all it is needed to be understood is WHY (and it could be not just one factor).

Map of average yearly lightning strikes per sq km from 1995 to 2013 (Credit: NASA)

But to return to this picture

and please take a good look. Notice the bright flash on the top left corner at the beginning that spreads into several leaders reaching for the ground. No ground path been established until two of those leaders reach the ground. From those two paths one of them has less resistance leading to a momentary current path among the cloud and the ground. I do think that in this case we see a Cloud To Ground lightning but of these two types, positive and/or negative lightning on an already established path. The image is taken from an High Speed or Ultra High Speed Camera, capable of 10k-1m frames per second, what we see is very slow motion of a cloud discharge.

Furthermore one thing you should have in mind is that lightning is not just a static electricity discharge.

This discharge may produce a wide range of electromagnetic radiation, from very hot plasma created by the rapid movement of electrons to brilliant flashes of visible light in the form of black-body radiation. Lightning causes thunder, a sound from the shock wave which develops as gases in the vicinity of the discharge experience a sudden increase in pressure. Lightning occurs commonly during thunderstorms and other types of energetic weather systems, but volcanic lightning can also occur during volcanic eruptions. Wikipedia

Above is written that the discharge itself may produce wide range of electromagnetic radiation. The majority of that radiation should be produced once the Cloud To Ground path has been established. In case we do have a combination of positive and negative lightning on that path then perhaps a differently filtered view of this lightning (microwave, infrared, ultraviolet, or even X-ray range) would have been more enlightening .

I actually don't think that all the radiation comes from the discharge itself as written above. It should be a combination of static electricity and radiation even before the discharge, only to be amplified at the moment of discharge.

An even more peculiar case than Ground To Cloud lightning is also the case of Ball Lightning. This video shows something that could be identified as ball lightning. One of the theories on how ball lightning occur indicate that Si element on soil could be a factor.

EDIT

In order to prove my point on this optical illusion in the above image i add a link of another optical illusion, of course on that video the train has one and only course but we see it to have both, in cases like that the human brain finds other ways to determine the direction.