Unlike any other rocket, the Atlas relied on pressurization for its structural integrity. Think of a truck tire: Aired up it has no trouble holding thousands of pounds as it takes a beating on the highway. Puncture the the tire and things go south quickly. Same deal with the Atlas, and in this case the Agena was the truck. And she was full of kerosene, which by a great stroke of luck happened to not catch fire before someone activated the fire suppression system.

Because complex systems often have mind-bogglingly simple failure modes, what you see happen in this video is all chalked up to the accidental introduction of a small gas bubble during fueling. Details are hard to come by, but there's a good account of what happened here:

http://www.thespacereview.com/article/1326/1