why do i suddenly start stuttering on words i said fine before?

why do i suddenly start stuttering on words i said fine before? for example, at work i’ve always been able to say “[place of business], this is [my name] speaking how may i help you?” but one day out of nowhere, my brain said i’m going to stutter on it. and sure enough, i did. what is the mechanism behind that? this has happened throughout my life. i’ll be able to say something fluently for years but one my brain just says i’m going to stutter on it now. it’s so brutal.