This could something as simple as tripping, or as serious as a cardiac arrthmyia.
It seems he may have had a seizure:
Chief Justice John G. Roberts Jr., a seasonal resident of Hupper Island, located off Port Clyde, was taken to Penobscot Bay Medical Center in Rockport mid-day Monday.
St. George Ambulance responded to a call at about 2 p.m. Monday of a man who had fallen 5 to 10 feet and landed on a dock, hitting the back of his head. The patient was ashen and was foaming at the mouth. The patient had reportedly suffered a grand mal seizure and was transported to Penobscot Bay Medical Center.
As of Monday afternoon, Penobscot Bay Medical Center would not confirm that Roberts was a patient.
Roberts and his wife, Jane Roberts, purchased their home on Hupper Island in June 2006. The island is located a few hundreds yards off Port Clyde and is only accessible by boat.
Roberts, 52, was appointed as chief justice by President George W. Bush and has served since Sept. 29, 2005.
More information will be provided as it becomes available.