Okay first of all ... Why vertical filming? Why ALWAYS vertical filming?!?!? Okay I'm better. Here's a video a guy took in Montreal of a fireball traveling down a power line. Its freaky awesome.

This was a high impedance electrical arcing fault. Usually the upstream protective device (ie. fuse, relay/breaker, recloser, etc.) will interrupt these types of faults and prevent them from continuing. They can start because of tree branches or animals or lightning strikes or anything that can bridge two phases or a phase and ground. The air becomes ionized and no longer acts as an insulator but a conductor. This arc is interesting because it was able to be sustained for such a long period of time.
If the fault has a high enough impedance, the current is actually not very high in comparison to regular faults. This makes it very difficult for the protective device to clear the fault since it the current can even be lower than the normal "load" current. The utility should be able to analyze any event reports in the area from the relays to try and determine what happened and if it can be prevented in the future.
Source: Electrical Engineer

Source Redditor BeesKnees21