I'm no expert but I stutter and I can only describe stuttering as a mental block, you are so caught up in trying to say the words you do anything to try and get them out! And also what you're saying can be applied to sounds for example the word 'reddit', one may repeat 're..re..re..' and it's easy to think you've said that part of he word move on to the next. But that's what stuttering is you simply can't, and that can be extended to words like 'fire' in your example. You just get stuck in a loop if that makes sense, repeating words, syllables, even phrases sometimes. With the singing, many people don't stutter when they sing my self included. From what I understand it's because when you sing, you take costal breaths, project, elongate words, change the tone of your voice etc all which aid fluency