Physics. Unlike virtually every other safe room manufacturer we use only Grade 8 hardened fasteners, much harder than standard bolts and fittings.
It's the same grade of hardness used when constructing bridges and high rise buildings. They are actually much harder than the steel panels used in the safe room, and that's by design.
Our panels are designed to be malleable to act like Kevlar...absorbing the energy by deflection, allowing the impact energy to be dissipated over time while the steel is deforming slightly. That's why 2x4's just bounce off our safe rooms rather than splinter into pieces....which is bad. Splintering 2x4's make great "wow" effect video, but it demonstrates the thicker, non-tempered steel has very limited malleability. If it is struck by an end-long piece of steel, like a piece of pipe or sign post, there's actually an increased likelihood it will be penetrated easier than our thinner, malleable panels because we soften the blow over time.
So....the bolts will never fail before the panels because they're much harder, and the storm shelter panels won't fail because they're tempered to absorb and transfer energy with the greatest efficiency. That's the same principle used in the "crumple zone" of your car...softening the blow by extending and dissipating the impact energy over time.