According to the Daily Show's Trevor Noah, the Surging Sanders is "when you come out of nowhere": a sexually charged pun based on Bernie Sanders' sudden appearance.
A term for ejaculating for no real reason, it comes from a candidate from the 2016 US Presidential Election, Bernie Sanders, going neck and neck with another candidate, Hillary Clinton.
Guy: I'm really excited we're gonna have sex again tonight!
Girl: I hope you don't pull a Surging Sanders again...