Peak torque RPM is the point where peak dynamic cylinder pressure is made, this is also where peak volumetric effiecientcy occures. So, anything that changes the RPM where peak dynamic cylinder pressure occures will change the torque peak RPM.
The single biggest influence to this is the cam, but anything that influences the airflow and/or combustion pressure will ultimately influence this. So, too big or small of intake ports can move the peak, as can too short or long of ones. Also, piston dwell, compression ratio, air/fuel ratio, quench, and exhaust flow (back-pressure or lack of) can all move the peak. Minor influences can even come from ignition, fuel grade/type, or by changes in physical properties of the combustables; Air pressure, air temprature, and available oxygen. All of which power adders contribute to and influence greatly. Generaly power adders increase power output without moving the point where peak torque RPM occurs much. Also, generaly the further the engine is from ideal for the given power adder the further apart the peak torque RPM points will be for that given engine with and without the power adder. (exceptions occur with engines built with a very low C/R for very high boost, or with engines that run a large amount of nitrous-amount than results in greater than 50 percent increase of total engine output)
The cam is the 'program' if you will, and the rest of the parts need to work with this 'program' to work to it's potential.
Most of us try to find a cam to match our engine combo, but in reality the cam choice should be made by our goals, and the rest of the parts shoud be ones that best work with that cam.