Muscle forms part of the tissues around these bones, but there is also a generous upper layer of fat both along the back and over the rump.  The first cause of diminished coverage to be ruled out is weight loss.  This is done by observing the body as a whole for weight loss.  In particular, are the ribs less covered by fat? (There is no significant muscular coverage on the ribs.)  Does the neck look thinner?  If the horse has obviously lost weight in general, this probably accounts for the changes in the topline as well.  A calorie count, evaluation of the digestibility of the diet and, if necessary, check for underlying disease states will locate the problem.