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Lightning cannot "loop", this is perpetual motion. It has to go down in potential. The looping is due to the suppressed third dimension--- the lightning is probably travelling towards the camera then down. –  Ron Maimon Sep 16 '12 at 8:10

2 Answers 2

I would put it down to coincidence: a cloud to cloud bolt concurrent with a cloud to earth bolt.

Have a look at this, where branching is also seen. Do not forget that in cloud to ground, a bolt starts from the ground.

enter image description here

Or this one, which shows following two branches in cloud to cloud:

enter image description here

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Lightning takes a largely random path. This is apparent if you watch the process before the strike:

enter image description here

Watch the video -- it is amazing. A somewhat less amazing illustration of the process:

enter image description here

This is from NOAA, via Wikipedia.

These leaders feel out a path between two objects (like the cloud and Earth), sometimes branching, traveling in complex paths. Sometimes, the path goes up, then back down, either because the strike is between clouds, or the random path just happens to take it that way by chance. NOAA's lightning pictures give a few examples:

enter image description here

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Viewed from the right angle, these can appear as a loop. The path is really more like a helix:

helix

but, viewed from the right angle, this can look like a loop.

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