It could be that a fox peeed on it because last year they did just that to mine and kept throwing themselves against the hedge, I actually stood and watched them do it, and now that part has turned brown and died, I was thinking of getting rid of it but at the moment the rest is growing well. Of course it could be something else !

Ours did this and it was honey fungus. scrape off some bark and if there is a white cotton wool layer or a very strong mushroom smell then thats what it is.