Under what conditions are passengers required to take precautionary measures during refueling? I was on a recent flight with a fuel emergency, during which the passengers were required to take precautionary measures. Under what circumstances are the precautionary measures mentioned below required?

The plane was diverted to a small regional airport apparently unequipped to service the Airbus we were flying in. Though the fuel truck had arrived at the plane and cones had been placed around the aircraft, we waited quite some time for the fire brigade to arrive before the fuel could be pumped. During this time and during the refueling operation, all the doors were opened (in the rain) and the passengers were required to be in our seats with no electronic equipment. Additionally, seatbelts were required to be unbelted.

The fuel emergency occurred while we were circled the Atlantic for 45 minutes waiting for the weather to clear up at Miami International Airport. When the alternate airport shut down as well (Fort Lauderdale International Airport) we were diverted to a small airport in West Palm Beach. From what I was told by a crew member after we had landed, we did not have enough fuel reserve to safely make it to Orlando.

Myself and other pasengers were under the impression that the in-the-seats-with-no-belts restriction is standard for refueling with passengers on board, but in the comments of another question I discovered that this is not the case. What are some likely reasons that these precautions were taken?

The incident occurred on 2014-05-15 on Iberia 6123 leaving Madrid at 11:50 with a scheduled arrival at Miami at 15:20. I'm pretty sure the plane was an A330 but it might have been an A320.
Doc 965, CAT.OP.MPA.195(b), regarding Refuelling/defuelling with passengers embarking, on board or disembarking, says:


  For all other types of fuel, necessary precautions shall be taken and the aircraft shall be properly manned by qualified personnel ready to initiate and direct an evacuation of the aircraft by the most practical and expeditious means
  available.


So no-belts restriction is one of the elements that ensures "expeditious evacuation". The same goes for keeping the doors open and keeping the passengers in their seats. 

Although the legal requirement is not that specific, probably the detailed procedure (with all the elements you described and others that were not visible for the passengers) can be found in the company Operations' Manual.