OPERATOR HAD FINISHED WIPING OFF FRONT WINDSHIELD & WAS ATTEMPTING TO REENTER CAB OF MACHINE WHEN HE FELL OFF. HE STATED THAT HE DID NOT KNOW WHAT CAUSED HIM TO FALL OR IF HE JUST LOST HIS BAL ANCE. HE FELL TO THE GROUND STRIKING HIS HEAD & BACK ON THE GROUND, RESULTING IN FRACTURED SKULL & 3 SMALL BONES FRACTURED IN BACK.