It's basically a misuse of verbage from the Town Hall form of voting, also used in many clubs and other organizations, where someone makes a motion (suggestion), and someone else will second (agree with) it in order to bring the issue to a vote.
person 1: I make a motion to accept this issue with the proposed amendment.
person 2: I second that motion
chairperson: All in favor say aye
It's a mispronounciation that just stuck, since you might actually agree with the emotion behind the particular decision which is being discussed.
dude1: dude, we should totally hit taco bell
dude2: dude, i second that emotion