An idiom which states an increasing amount of danger or tension, similar to "the shit's hitting the fan" only much more terrible and apocalyptic. The phrase is partly ironic, as if to exaggerate the rather mild (though hilarious) premise of the "Snakes on a Plane" movie starring Samuel L Jackson, which has snakes on a plane.
"How bad was the motorway pile-up?" "Let me tell you, it was like snakes on a plane down there."
"How's your relationship with Chloe going?" "It's getting close to being like snakes on a plane man, seriously." "Damn, dude."