Here’s the same thing said differently:
Under some conditions a truck can easily go into a skid.
If the truck is going straight, it’s possible the tractor
and trailer will stay straight and, possibly, no damage
will occur. This is unlikely. Most likely one set of
axles will begin to skid before the others, that axle will
have less traction than the others, and most likely the
rig will not stay straight.
If the steering axle locks, the tractor may still remain
straight, as the front wheels are sliding and the rest
are holding traction. The driver has little steering control.
If the rear tractor axle locks (the axle in the middle
of the rig) most likely the rear axle will begin to
rotate around the kingpin causing a tractor jackknife.
If the trailer axle locks (the last axle in the rig)
the loss of trailer traction will probably rotate around
the kingpin and cause a trailer jackknife.
All of these result in a loss of control and probable
accident. If the load shifts on the trailer, everything
gets worse. A liquid load is always shifting, so pulling
a tank with liquid can be interesting.
Of course, all the axles can lock, also causing a
jackknife. Most rigs have anti-lock brakes, nowadays.