Nothing to do with its breeding it is just a somatic mutation (a one off and wont pass on to progeny) very flash, we had a die laughing x ollies magic filly with one that went from the inside of her knee round to her elbow and then completely covered her girth and right up to her wither, you often see them in grey arabs and they are refered to as a "bloody shoulder" as its exactly what it looks like on them red hairs on a white/grey coat and on a bay they are refered to as a "reverse bloody shoulder" as they are white hairs on a dark coat.
I prefer the pintos and dominant whites at least their colour can be passed onto their progeny but sadly rarely as well bred as this one, except in the case of the dominant white courage and art major colts but they are one in a million