Question: why do some stutters manifest themselves as repeating words?

Question: why do some stutters manifest themselves as repeating words? I'm watching a youtuber who has a stutter, and I noticed that as well as having trouble getting a word out (f-f-fi-fire) he sometimes has trouble getting a compound word or two-part word out (fire-fire-fire-firetruck). I know it's never this simple with something like a stutter (or people wouldn't have stutters), but I want to ask, if he can get "fire" out in one go, why does he have to repeat it several times to get the second part "truck" out? Why not stop after successfully saying "fire" the first time, and then move on to try "truck" as a separate word? He once explained in another video that he doesn't stutter when he sings because the part of the brain that recalls a memorized song is different than the part of the brain that has to conjur up new sentences. So I assume it has something to do with how the brain works, but I'm really not sure. I don't know much brains either! I don't have a stutter, myself, but I'm curious and I figure who better to ask than the people who know it best?