Night of the Twisters

The film's prologue takes places in an area of rural farmland in Dannebrog, Nebraska. While there, Bob Irisen â a storm chaser with the National Weather Service â is driving down a country road to track a freak supercell thunderstorm. As a tornado touches down yards away, right as their daughter arrives home from school, Bob warns a family about the oncoming twister, which sends them running into their cellar just before it destroys their farm.
Meanwhile, in (the fictional town of) Blainsworth, Nebraska (the book is set in the real-world city of Grand Island),[1][2] teenager Dan Hatch, an aspiring and practicing artist who is constantly being pushed by his stepfather Jack to be an athlete, participates in a bike race and damages his bicycle; Jack pushes Danny into finishing the race on his bum bike, where he falls behind the other racers. Much to his luck, he wins a new bike in a raffle held by a local bank. While trying out his new bicycle, Danny and his best friend Arthur Darlington run into Arthur's two sisters, Stacey (whom Danny is infatuated with) and Ronnie Vae, while at the park. Danny and Arthur arrive home as the former's mother Laura is preparing dinner, when she also asks Danny to tend to his baby half-brother, Ryan. Later, Laura's sister, Jenny, calls to inform her that they have been assigned to a fill-in shift as waitresses at the Salty Dawg, the local diner where they both work.
On the road, while continuing to track the massive storm creeping toward the town, Bob decides to head southeast into Blainsworth, as the meteorologist he is radioing to is astonished at the uncharacteristic spring-like nature of the storm system for the fall; while there, he meets Laura and Jenny while getting some coffee at the diner. The action begins after Jack comes home at mid-evening; as he sits down to watch television, the show he is watching is interrupted by a severe weather bulletin reporting about a tornado sighting near Dannebrog, 14 miles (23Â km) northwest of Blainsworth. Several minutes after Jack leaves to check on his mother Belle, who is asleep in her rocking chair as he phones her, tornado sirens blare throughout town, only to cut off abruptly as Danny goes to get Ryan from his crib. The eerie stillness outside afterward suddenly gives way to a violent tornado that approaches Blainsworth's Capital Heights neighborhood, with the sucking noises emitting from the drains notifying Dan and Arthur of its pending arrival; they and Ryan take cover in the basement bathroom's shower as the twister starts to obliterate the Hatches' home.
After Dan and Arthur escape from the basement of the leveled house through the collapsing floor beams where the first floor once stood, and look in awe of the rubble that was once the Hatch family's home, Arthur runs into Stacey and Ronnie Vae, who both survived the twister themselves in the Darlington's home (their parents were out of town on a trip back home to California at the time the storm hit). As Dan struggles to find his own family, Laura and Jenny are trapped inside the Salty Dawg, which also gets destroyed by one of the tornadoes. Dan and Stacey then go save Dan's grandmother, Belle, at her farm; the two teenagers find Belle underneath wooden boards blown onto her from off of the partially damaged barn near her house (which itself survived intact).
As Dan and Stacey rush in the car to get Belle treated for her injuries, Dan finds Jack on a closed road, with his truck â which was overturned by the tornado, pinning him underneath it â covered in fallen power lines; he pulls Jack out from under the truck (with the help of emergency crews, after an earlier attempt by Dan and Stacey to push the truck in order to free him nearly injures Jack further), seeing this as an opportunity for his father to finally see him as reliable. However, Jack just gives a simple thank-you to the fact that Dan saved him from multiple dangers, even though his stepson may have been the only one down the road who was able to help him as it was blocked by policemen due to it being blocked by the downed lines, debris and broken underground utility lines. Later that night at the shelter, Dan reveals to Stacey that Jack is his stepfather and that his real father, Daniel Sr., was a pilot who was killed in a plane crash when Dan was 6. After telling her that he feels he isn't good enough in Jack's eyes and talks about the good qualities that his stepfather has, Stacey helps Dan consider that the two could try to find some common ground.
Eventually, Jack â followed by Dan, who sneaks himself and Ryan into the station wagon loaned to Jack â leave the shelter to go and look for Laura. Just as Bob pulls his truck into the driveway of the destroyed house, helping passengers Jenny and Laura along with him searching for Jack, Dan and Ryan, Jack's station wagon also drives up and the family is reunited. As soon as everyone is relieved they survived the storm, three tornadoes touch down near them. Bob, realizing that there is no adequate shelter available, advises the group to make a run for it in their vehicles. The group narrowly escapes one of the twisters, which picks up a car that the Hatches' neighbors try to outrun the tornado in themselves, destroys several buildings in its path and hurls a tree branch into the windshield, briefly knocking Jack unconscious â causing Dan to take over driving the vehicle out of the storm's path. They, along with Bob and Jenny, make it to an overpass as the twister blows out the back window of the station wagon, nearly sucking Dan out before it dissipates into the air. As they walk out from under the overpass just as the sun rises on a clear day after the storm passes, Jack admits he's proud of Danny for not giving up in the face of adversity and trying to reunite the family.
In the film's epilogue, illustrating what happened with the characters one year after the storm, Dan explains that he is now dating Stacey, while Arthur became class president, and Bob and Jenny got married and are becoming first-time parents to twins. He and Jack have also become closer, and Jack is now supportive of him. He also tells that his grandma, Belle, died the following year in 1997.

Question: What caused Jack's truck to overturn?
Answer: tornado