pistons move in a straight line motion, rods move in an up/down and circular motion. the forces affecting these parts are not torque as torque is purely a twisting motion.
if you want to break it down to the basics the main thing here is cylinder pressures, the amount of force placed on a piston causing it to move through it's travel. good tuning keeps the pressures fairly even between cylinders, but as you increase HP, you're increasing the amount of pressure in the cylinders (more air/fuel=stronger burn=more pressure). the piston is pushing down on the rod when the cylinder fires and then the rod translates that motion into the crank to create torque. at a certain level, the force placed upon the rod will cause it to bend/break or you can shatter/blow a hole in a piston. in a perfect world these consistant pressures would make the engine more reliable as long as they are kept below the breaking point of the parts in the engine, but this is never the case.
this is where detonation comes in, because it causes huge spikes in the force upon the internals. preignition/detonation causes the air/fuel to burn while the piston is still traveling upward being pushed by the rest of the engine, so it's kind of like the difference between running your car into a stopped semi truck at 30mph or hitting the same semi truck head on when they're coming at you at 60mph. the forces placed on the parts are multiplied, and then suddenly your nicely tuned engine becomes shrapnel. the counterpart to this is heat, because as pressure increases so does the temperature (why tune by the EGT gauge
). extreme pressure causes extreme heat, which compounds the problem of bad tuning causing burned pistons/valves if you're lucky enough not to throw a rod through the block.