Given the speculation on jet fuel quality at BJS, my personal theory is that on attitude change on finals (aircraft nose up with flaps extended/gear down) a volume of free water moved in both of the wing tanks to the boost pump inlets and sent a slug of water down both engine feed lines.
Free water would have remained frozen to the bottom of the tank for most of the flight, but defrosted on descent. The water scavenge system is quite good on a B777 though...
It's a tribute to modern aircraft crashworthiness that there have been 3 recent landing incidents resulting in aircraft off runways (AFR A343 Montreal, IBE A346 Quito, BAW B772 LHR) without a single serious injury.