How about when he is running down the stairs and grabs the rail? Where is the knife? Something about this story does not pass the smell test.
Don't everyone fly of deep end now. 
I actually watched this video about 100 times trying to see when the knife first appears. It does seem like he pulled it out of his pocket. Now I was watching to see how the knife ends up open, as it's a butterfly knife it takes a certain wrist flick to open up. If you watch closely, it does look like right after his hand reaches into his back pocket (he then passes the camera blocking the shot) that his hand does a double wrist flick and then you suddenly get a clear look at the open knife.
It may be because of the angles and unclear video that you can't see it up until that point, but to me it does look like that is when he pulls it out.