They ahve decided that a "dislodged shoe" was the reason Big Brown came in last.

According to The Daily Racing Form chart, "Big Brown steadied and broke outward [from post position 1] at the start." At that point, another shot by Mayberger, taken from behind, reveals Guadalcanal's left hind hoof stepping on Big Brown's right hind as the favorite comes out into him. Big Brown's head jerks sharply left at the moment of contact as Kent Desormeaux rises in the irons and shifts his weight to the left.