The arrow was probably broken by the buck in a frantic attempt to vacate the premises. The hind legs came forward at the shot and broke the shaft off as the leg muscles bent it against the ribs. A lot of the time you will actually hear the arrow snap as it breaks. It will also happen with the front legs. This is not the issue though, the deer has a fatal wound and will die. Gut shot deer will lay down very soon, within 100 yards, after the shot but they may get up and travel again due to the discomfort or from being pushed because of tracking too early. You can usually find blood and stomachor intestine contents though! I found a buck several years ago after tracking 12 hours and finding very small drops of blood on vegetation. Mark your backtrail with toilet paper to make it easy to come back to the last blood found. Move forward, sometimes on hands and knees, to find the next blood drop. Keep it up if it's not been too warm. Good luck!