It is a genetic abnormality that causes the shoot elongation with multiple lateral shoots arising on the same branch. It creates a flat branch and is not harmfull to the plant. Personally, I think it looks pretty cool. I can't find the technical term for it at the moment but I am sure one of the members here will. I have seen it on fruit trees a few times and I usually keep the branch piece with me to show to people as it is an interesting conversation piece for plant nuts. For a better view can you take a picture from the "side" to show how thin the branch is?
This kind of mutation is called fasciation. It is extremely common on Forsythia. For more information, see this review article Occurrence and Causes of Fasciation by Pierre Binggeli (Cecidology 5, 57-62, 1990). If you were wondering, Cecidology is the name of the Journal of the British Plant Gall Society.
