RROD is usually caused by a processor or RAM failure. What happens sometimes is that the constant heating and cooling of the chips can cause the solder balls to fracture and break. When this happens the chip effectively becomes disabled.
To avoid future RRODs, keep your xbox cool (avoid laying it on carpet, put it somewhere it can "breathe" and do occasional cleanings of the cooling components) and if you service any of the cooling components, make sure you use a high quality thermal paste like [product|IF179-010|arctic silver]. You can follow our [guide|744|thermal paste guide] if you need more information on "how-to".