Don't need to know WHY the curve happens, but an easy way of thinking about it is relating it to 'probability'. A 4000K black body is a black body with an average temperature of 4000K, but there may be areas which differ from that temperature. With a certain temperature, there is an associated energy, this energy is high enough to allow the 'electric oscillators' to undergo certain transitions, the peak in the BBR curve corresponds to the transition which is most probable, however, in areas that are a little hotter (i.e., have higher energy), higher energy transitions (lower wavelength emission) can occur, but the probability of these transitions is significantly lower as there would not be many regions which have this energy. The lower the wavelength (higher the energy transition), the less likely it would be that the blackbody surface will possess enough energy to allow the transition to occur, hence why the BBR drops down to zero quickly as you move towards a lower wavelength from the peak wavelength