You're blocking because you're holding back. Because you can't just let go and stutter the word out. Even if you think and try really hard to, you body doesn't let you. Your want to speak so you go forward at it, but then there is this other side, usually something deeper/subconscious that says it's not alright to stutter that gives this equal counter force backwards. you push forward, it pushes back and boom, you're caught in a block. As to why we stuttered on words A, B, and E but not C, D and F...well I dont know either.