Jackknifing is a situation when a two-part vehicle like a tractor trailer has one end swing around at an angle like the shape of a closing jackknife. This can happen due to inexperience with reversing or due to weather conditions.
Jackknifing is a more common problem in wet or icy conditions when the wheels lose traction, causing them to swing out of the driver's control. This is particularly the case when the trailer is empty. With less weight, it is less able to grip the road properly. When inexperienced drivers try to reverse while towing something, it is not uncommon for them to also experience a jackknifing situation. Drivers should research tips on driving while towing before going out onto the roads to avoid situations like this.