"Jumping the shark" is an expression which means that something has reached its peak and can only go downhill from that point. It comes from an episode of Happy Days, in which Fonzie actually jumps a shark on water skis. It was a sign that the writers were desperate for ideas and that the show was past its best. It is now widely used as shorthand to express that something has run its course.
"Jumping the couch" is much more recent, and was probably inspired by "Jumping the shark", as it is also a defining moment where everything goes wrong. It refers to Tom Cruise’s antics during the Oprah Winfrey show, where he manically bounced on the couch to express how in love he was with Katie Holmes. Hence now, this expression is used to describe the moment where a celebrity has lost credibility through a moment of madness.