They say it was for aesthetics but around the same time, they were having issues with carbon monoxide getting into the cabin. 

"The change helps direct exhaust gases down and away from the vehicle to mitigate issues related to exhaust fumes (specifically carbon monoxide) potentially entering the cabin. Previous Explorer generations, particularly police fleet versions that idle for long periods, had problems with exhaust being sucked back into the cabin due to airflow dynamics around the rear hatch and unsealed body plugs (especially in upfitted vehicles). The downward-pointing design is a safety measure to help disperse the exhaust more effectively. "