Why do horses get sudden white spot on there shoulders?

This is usually a sign of an ill-fitted saddle. If the spots are occurring under the saddle blanket and in an area covered by the tree of the saddle, this is almost certainly the cause. Investigate proper methods of saddle fitment. The most useful reference book I know is "The Horse's Pain-free Saddle Fit Book" by Joyce Harman. I have used this book and can recommend it.