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? Perhaps, who knows if one did not examine more thoroughly.
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 negative lightning on an already established path.
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
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 probable source.