Volume faders moving to -∞ by themselves

HI all,

I am using Logic Pro 9.1.8 on my iMac, an audiobox USB interface, 2 midi usb cables; one to my nord electro 2 and the other to my v-drums. More often than not if I am playing my v-drums or something midi while not recording the volume faders will just shoot down to the bottom (or to -∞) I can't figure out why this is. Any help is greatly appreciated. It's pretty annoying. Thanks.