What is the difference between a 'tractor jackknife' and a 'trailer jackknife' in terms of mechanical cause and vehicle behavior?

1. Tractor jackknife is caused by steer tire failure; trailer jackknife is caused by fifth wheel failure
2. Tractor jackknife is caused by drive-wheel lockup, causing the cab to spin; trailer jackknife is caused by trailer-wheel lockup, causing the trailer to swing out
3. Tractor jackknife only occurs when backing up; trailer jackknife only occurs while driving forward
4. There is no difference; they are identical terms for a trailer breakaway