"Сейчас" in Russian is not only used with present, but with future tense also. Think of it that way. "сейчас+глагол в будущем времени" is an equivalent of Enlgish Present Continuous tense.
When you say, for example: "He's going to fall now". Here, although in English you use a kind of present tense, you actually talk about a near future. In Russian, you we say the same like this: "Он сейчас упадет".
If I said "Он сейчас падает" it would mean that "He is falling RIGHT NOW", in this exact moment he is in the process of falling.
So, "сейчас+глагол в будущем времени" means that this something is going to happen in the next moment.