I tend to make my players stay very concrete and observational. Absolutely no causal explanations for the apocalypse allowed, gently and politely denied.
-What do you see when you look to the horizon at sunset?
-What smells waft to you on the wind?
-What wakes you from your daily slumber?
-What's the best meal you've had this month?
-You've just caught yourself staring off when your eyes suddenly focus...on what?
-A chill went up your spine, you know a threat is just outside of your senses...what is it?
and so on.
The players responses stay more or less independent at first, but quickly start to intertwine, begging for explanation.
The inputs are like points of information, with each player developing hypotheses to thread the points together in explanation, but I won't let them go there.
Playing into the confirmation bias, players will then add more content consistent with their own hypothesis and the group starts a slow build towards something for us all to discover together.
The momentum is established once the players start stepping on one another for air time. I go Socratic, picking at details to help me with threats and such.
Sometimes, I need a clock to cut off the rolling content flow that occurs. Maybe an hour or so.
Then it's time to discuss how the group fits into this world they've created.
Fun thing is...I never explain what or why the apocalypse is. It just is and no one knows.