I'm going to guess that the answer to this question is "just because", but here goes anyway. Why is the "c" in "seconde" pronounced as a "g"?
It's not really a "g" sound some much as a swallowing of the "c" sound. I dounbt that helps much though.
It has been change a long time ago because of the way the sounds uses are vocal cords. The c [k] sound doesn't make our vocal cords vibrate has opposed to the g sound. And because the o is a vibrating sound it's easier to pronounce two vibranting. So the real answer is laziness ;)