What are possible explanations for fuel being expelled in 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.