Rust is a likely cause, too, though one can't be sure without looking at the subject vehicle in person. Usually a splice repair has the opposite issue - it's TOO strong and messes up the controlled crush (designed collapse of the body structure to protect the occupants).

My tendency is to add gussets and stiffen the body which is great for daily driving but bad if the vehicle is involved in a collision. I'll wager that most professional body guys have to fight that urge, too, though modern repair practices warn against over-strengthening the structure. As noted, it's also difficult to tell what was done to extract the occupants vs. what failed in the collision. Either way, it's better to keep your car and trees (or other immovable object) at a greater distance than these two drivers did.

Dan