When the key contacts underneath a B3 note come together upon playing a key, an electrical circuit is completed. And when that happens, a short electrical "spike" is created as electricity rushes through the circuit. This ends up being heard in the sound. It's not unlike having a light switch in your house which, when you turn it on and off, causes a pop to be heard in a radio.
Some B3's exhibited more key contact noise than others (the amount of noise might increase with wear as well as the degree of amplification). Some players like this sound more than others, so the amount was made controllable in the physical modeling of the B3 plugin.
