I've had to evacuate for fire - both real and imagined (fake alarm pull). It could be a quake. It could be a sudden water main break. For now let's not say a levee burst - chances are in a situation like that the show would be cancelled. It could be a masked gunman or an explosion. Let's say for the sake of covering all bases that yes, you're evacuating the whole building or at least the theatre wing. As a variant, read up on the 2002 Nord-Ost crisis and come up with a plan for dealing with that. If it's happened before it can happen again. As this is an academic exercise, take it as far as you feel comfortable.
I would say in general, if you don't have a trained house manager then think about the audience too - or at least figure out, if not you, then who would take care of the audience members, and what you would do to ensure the house is clear if you believe you'd have any responsibility for doing so. Regardless, figuring out the chain of communication on that front is definitely crucial. Does the HM only go to rendezvous point once everyone is out? What if they don't come out?
Thanks for supporting the idea as important, late_stranger.