That's about 15 gallons per minute if I did my conversion right. That's a pretty big leak - not a leaky coupling o-ring. There are a few things it could be, depending on the airplane model. What model was it? Were they feeding from the center tank at the time or did it start right after center tank runout? The one event I've seen like that was eventually attributed to a center tank pump outlet check valve jammed open at pump shutdown by a piece of debris, allowing some of the output of the main tank pumps on that side to flow backwards through the center tank pump into the center tank