There are a few types of turbulence that can cause such a major reaction from an airplane the most likely being clear air turbulence (CAT) of which there are a few types.
This is the wingtip vortices caused by the pressure differential around the wing that come off an airplane and spiral down and away from an airplane. Generally, the larger an airplane gets, the more wake turbulence it generates because of the increased lift needed for the plane to fly. These vortices extend down and behind the airplane for a significant distance and can cause another airplane to lose control. There was an incident a few years ago between an A380 and a Challenger 604 where the Challenger crossed below the Airbus that resulted in such a situation. This one is probably the easiest explanation for your script as it can be very severe and the mechanism for generating it is a bit more simple than the weather-based turbulence types.
Jet Stream Turbulence
A lot of airplanes use the jet stream to get favourable tailwinds in order to get to their destination faster. However, there is a good risk of turbulence occurring due to the large wind differentials between different sections of the jet stream. The area of most turbulence when flying in the jet stream is just under and on the polar side of the jet stream core. Typically, the turbulence isn't as severe as what is described in your script, but it can occur (example).
Mountain Wave Turbulence
When wind passes over a mountain range perpendicularly and with a few other weather conditions, it can generate mountain wave turbulence which is a large oscillation in wind patterns that can extend for very long distances under the right conditions. This type of turbulence will have areas of major upflow and downflow which can change an airplanes altitude significantly. This type of turbulence can combine with jet stream turbulence to create even worse turbulence.
There are other types of turbulence that can cause severe turbulence like thunderstorms, but those are much easier to avoid as they are visible (not the turbulence, but the clouds associated with it).