What is a 'trailer jackknife' and how is it caused?

1. The tractor's drive wheels lock up, causing the tractor to spin around the fifth wheel
2. The trailer's brakes lock up, causing the trailer to slide sideways and swing out of its lane
3. The kingpin shears off, causing the trailer to slide forward over the tractor cab
4. The landing gear collapses, forcing the trailer nose to hit the road surface