Without too much speculation and due to the state of the vehicles/impact damage I would say speed. Big question for me is: Why the loss of control? They had probably done this journey many times and knew the road.
What does grieve me is the fact that the couple in the taxi did the sensible and safest thing after a night out. They got in a taxi to get them home safely, and the taxi driver was earning his living providing a service......
I saw pictures of the crowds at the taxi drivers funeral, so sad and those poor children will never see their daddy again.