Simple logic, plus about 50 years of being around frame straightening! Anything and everything in the area of a wreck impact gets stressed as the crash damage radiates away from the impact point. Unless there's a gross bend or a wrinkle, the damage isn't always apparent until an undamaged part is compared to the crashed one. Getting a frame on a dirt track race car straight after a pileup on the track was a real exercise in frustration, especially after the car became the "meat in the sandwich" in a 3-car stackup at the end of a straightaway!
Jerry
