x4 on what was said.
If you look at that cage, there are no members which prevented it from folding side to side, which is what it did.
Triangles require a member to bend for a triangle to fold. So the strength is created by the structural member. Looking at that cage from the back, its like a big rectangle. All of your strength now is only in the joints.
An X-brace behind the rear seats would help and side-to-side integrity.
Its bent, yeah, but you would still have been fairly well protected by that.
Also, If you are looking for a strong cage, then ditch the stock parts, and have it all bent up from the same material.