Posted: 17/05/2015 at 14:20
I suspect it might be a fungal infection, let in because too long a stub (aka 'snag') was left when the diseased branch was removed. Branches should be cut flush with the collar, so usually only an inch or so of stub is left. If the stub is too long, the bark can't grow over it to heal the wound. See the RHS advice on this:
and more detailed advice with images here:
I hope I'm wrong.