How to Write a Climax Scene

If there's one thing I hate, it's when I get to a climactic scene and now I am told somebody's motive for being in this scene. That is as close to Deus Ex Machina regardless of what the outcome is. When a character makes a significant action or a decision, the reader wants to have all the pieces already so that they can know exactly why the character did that. You know, I'm constantly befuddled by this. I teach creative writing to new students. And how many of them won't use the thought tool?