The above linked articles explain this in more technical ways, but here’s my laymen’s explanation. When I was very young, I was a stutterer. Even once I overcame stuttering, I still have always struggled to verbalize my thoughts (I’m much better at writing them, in fact I’m very good at writing them).
Whenever asked, I explain it like this: While I’ve never been outwardly hyperactive, my mind is more hyperactive than a kid on a pound of sugar for the first time. It runs at 200mph while my body can only go 55mph. So, my mouth struggles to keep up.