Why does lightning do this? Well, for starters, that’s only one of the main types of lightning. There’s intra-cloud lightning, which stays in one cloud, jumping between different charge regions; inter-cloud lightning, which hops between the clouds; and cloud to ground lightning, which does what it says on the tin. So, while we’ve known about the many zigzaggy kinds of lightning, we’ve had no idea why they get all jazzy going from A to B - surely it would be more efficient to just go straight?
Researchers now think all of these cool forks are caused by these little guys: singlet delta oxygen molecules. When lightning is forming, the electrons within the thundercloud hit oxygen so hard it makes a bunch of these particles, which then form into highly conductive columns. These columns redistribute the electric field and successive bursts, so the lightning ends up forking and making all those steps.