I am experienced presenter, my job is to show presentation, organize educations and trainings to different people, with different personality traits in different situation. Until now I collected more the 700 hours of presentation and more then 120 hours of presentation education. In the beginning I had also that problem, but I overcome it by exercise. Now I am complitelly relaxed when present.
In my opinion cause of this is social anxiety in self presentation:
Here is one research, but I am sure you can find several more if you look to literature section.
Also people get aroused because if they think that other judge them, especially if they do not present a lot... here is model how people conclude others judge them
EDIT: In this case 'fast talking' is indicator of public speaking fear, so you cant using 'public speaking fear' as cause of 'fast talking' because it will be circular definition. Here is a try to explain this in S-O-R paradigm:
I dont know exactly what is happening in brain but I assume that something is happening in girii hippocampy (working memory) which is overloaded with information of judging environment and accomodate presentation, but also have significant role in speech production.