Leaving all engines running at the gate

Recently at SJU I saw an IB A340-600 arrive from Madrid that left all
four engines running during the 2 hours that it was at the gate before
continuing to Santo Domingo. After all the passengers got off the
entire crew, both cabin AND cockpit crew got off and went to the
transit lounge for about an hour before returning to board for the SDQ
leg. All this time the engines were left running. In fact, they
never stopped running the whole time they were at SJU. I have never
seen an airliner do this before and was wondering if something was
wrong with this particular aircraft or if this is some kind of new
"windmilling" feature on the new A346 or something. Perhaps related,
I've heard that IB have had a horrible experience with their new A346s
and this particular aircraft, EC-INO (Gaudi), the first one they
received, is said to have given then more headaches than all the
others.

What circumcstances would make an airline leave a plane guzzling gas
for such a long time on the ground?