The original meaning of the adverb momentarily was "lasting for brief time": "She was daydreaming and momentarily lost her train of thought." Today, the word is frequently used to mean "very soon." A waiter might say to a hungry customer, "Your food will be ready momentarily."
When something is just about to happen, it will happen momentarily. Just before a concert begins, an announcer will often tell the audience it's starting momentarily. But if you pause your movie momentarily so you can go grab a snack, that means you will put the movie on hold for a short time. The "lasting for a moment" meaning of this word is attested from the 1650s; it wasn't used to mean "at any moment" until the 1920s.