What type of event or decision would cause the market to Crash?

ok, so first of all-mods if this in the wrong area please move to the right one. Didn't know for sure where to post this???

Now- I was just curious, what type of disastrous event or "inside" decision would cause the Stock Market to crash? We all know that after 9-11 the market plummeted...I know there has been a lot of talk lately on threads in regard to something like this happening soon. For example here are two of the latest threads regarding this:

www.abovetopsecret.com...

www.abovetopsecret.com...

Now that we are approaching September 21st this week (I believe a predicted market crash date?) I am worried that something cataclysmic could occur in the following days...What do you all think? I thought this could be a great discussion to have...