Yes, I know why that happens. It has a difficult neurological explanation but it comes to this: When you sing, you automatically use the respiratory muscles, the intercostal muscles, and the diaphragm in the correct way. They are engaged in a certain way, causing the diaphragm to always move smoothly upwards. That’s why people don’t stutter when they sing, because then the diaphragm always makes a smooth movement. So, you need to learn to apply the singing breath to speaking. Just like opera singers do. This can be achieved through specific exercises that train the intercostal muscles. Eventually, you will be singing and speaking at the same time. Look, I’m still speaking fluently now, because I’m singing and speaking at the same time. But I have the singing breath. But you have to learn that, you have to relearn it completely. It cured my stammer 100%.