It died because it was near death. Think about it, the pressure difference between surface and 2,000 is immense. The videos are not accurate representations of how it swims as it would be completely different under thousands of pounds of pressure. The only reason a creature like this would be seen is if it had been caught in a fishing net and the capt being scared of being caught with it, tossed it out of the net. It would never survive. Or if it was sick enough it might head up but in reality it would have died deep below and have been eaten before surfaceing. It's amazing what death occurs in the fishing trades.