If the temperature of the unburnt mixture exceeds the self-ignition temperature of the fuel and remains at or above this temperature during the period of pre-flame reactions (ignition lag), spontaneous ignition or auto-ignition occurs at various pinpoint locations. This phenomenon is called knocking.
Engine Speed: increase in engine speed increases the turbulence of the mixture considerably resulting in increased flame speed and reduces the time available for pre-flame reactions. Hence knocking tendency is reduced at higher speeds.
Load: As the load on the engine increases more amount of fuel is inducted into the engine cylinder, the temperature and pressure increases.