NASA is anxious regarding the fuelling procedures of Space X. This Fear arose after realizing that fueling is an extremely dangerous operation that shouldn't be practiced anywhere near people.
Space X previously has successfully landed their Falcon 9 rocket on a drone ship at sea. They even announced their intentions and plans to send humans to Mars with an elaborate and detailed press meet. But, NASA has pointed out a very dangerous situation here. They have encountered accidental disasters in the past, that claimed lives of astronauts.