In another question, I asked about whether or not a fuel spill would be reportable or not. The answer was "no", because the aircraft was not in flight.
What are the possible causes of the spill demonstrated by this video?
I can think of 4 possible causes, 3 of which can probably be discounted.
Tank venting. The fuel system is designed to deliberately "leak" if the tanks are overfilled or, the fuel expands due to temperature changes. Whilst I haven't seen this happen on a B767, I have seen it on numerous other types and when it happens, it does not gush out like it does in this video. It's more of a seep of fuel which drips off the underside. Whilst it can be a lot of fuel, this video does not fit a venting scenario. Additionally, the flight crew would be very familiar with venting which would be a normal event and would not have abandoned the flight.
A fuel leak such as a ruptured pipe, a failed seal and so on. I would expect to see fuel ejected like this but only from a pressurised part of the system, downstream of the fuel pumps. The location of this leak is not from a pressurised part of the fuel system and in my opinion, there is simply too much fuel for it to be a result of gravity. It also appears to be coming from a nozzle rather than seepage
As can be seen in this video, the source of the fuel is coincident with the location of the fuel dump nozzle. It is possible that the crew accidentally activated the fuel dump but unlikely (inconceivable?). The switches are very clearly surrounded with "don't touch this" markings and require two separate switches to be activated. Additionally, the jettison valve switches are "guarded" such that you have to lift a spring loaded flap up in order to be able to press the button.
The APU draws it's fuel from the left main tank. This flight was close to the maximum range of a 767 so would have been at or close to maximum fuel load. If the APU had been running for a long time, longer than expected, it's possible that the crew started a fuel transfer to balance the tanks by pumping fuel from the right tank to the left tank and forgot to switch the transfer off. This could have pressurised the left tank causing fuel to overflow into the surge tank and vent out (the surge tank vent is also very close to the jettison nozzle). I believe that this is the most likely scenario.
This is the only authoritative source I could find showing the fuel jettison switches.
All supposition of course but a strange incident. I'm sure there may be other possible causes and I'm sure that comments and other answers might expand on this.