Sweet Crab Apple Tree

Took my car in for MOT this morning. No problems and a free valet service and car wash thrown in for free. Came away a happy girl!  It's been a cold, dull day (10 degrees) but there was a brief sunny patch this afternoon that shone brightly on  my crab apples.  What I don't understand is why the leaves are turned so that the backs of them are facing the sun.  any ideas?  There has been quite a bumper crop this year - plenty for me and the blackbirds!