I just spoke to a friend who was there - in Jarabacoa - and spent an hour in the traffic at the accident.
Apparently, it was the driver's first trip on that route. Experienced drivers keep the bus in low gear for the descent, this fellow switched gears and lost control - hence, CB's no skid marks observation.
One of the passengers remarked on the gear change - it wasn't her first trip, I guess
All in all, sad news but harks back to the better training suggestions made here