The rest of the pileup is varied. Some of it is from too short following distances or really lack of reaction.
Some drivers in the back had no ability to see the pileup ahead and had cars stop in front of them (from dissipating their energy in a collision) in less distance than they could brake.
In other words, these drivers did not leave enough distance to react and brake.
Again, recommended following distance is ~250-300 feet when stopping & reaction time can be 350-400 feet. This is OK when the car in front of you brakes, because you'll both be decelerating with a similar profile and you'll have more than 250-300 feet to stop. It is not OK when the car in front of you stops substantially faster than braking with no warning because it has hit an unknown obstacle.
That explains a two-car collision. The following pileup is explained by drivers not leaving enough space to react and brake.