Computers (and an Android phone is a computer) do things behind the scenes. You barely notice it when you're reading emails or something like that, but with music or video, if the phone runs a background task and doesn't have enough power to also keep the music running, you'll hear the pause. Even the fastest PC will do that sometimes.
(You could have a lot of apps running because you installed them. You could also still have apps running because you "backed out" of them by using the Home button instead of the Back button. That leaves an app running in the background, it doesn't close the app.)