The "Willie" portion of the name originates from "Willie" Nelson the famous country singer who smokes marijuana. This is because much like a person who has smoked some mary-jane the enemy team is happy that they have found a lone person to take out for an easy kill.
The "Nixon" portion of the name orginates from President Richard "Nixon". This is because the whole thing is a lie much like Watergate. The lone player is not an easy kill, but a trap.
"Set-up the Willie!"
"Don't go in there it's a Willie Nixon!"
"There's no escaping a good Nixon."
"Pull them into the Willie."