I think you may be experiencing a reaction to some form of stress. It is well known that peoples pitch, tone and speed of talking can change when suffering with anxiety, anger and stress.
I think some of this comes from the adrenaline burst that often accompanies it. Adrenaline gears you up for a perceived emergency and hence can make you think faster, move faster and react faster. So speeded up speech might be happening because you are geared up for what your body perceives as an emergency of some kind, so your thinking will speed up and perhaps your speech too and the need to get your message across as a result.
However you can overcome this. Perhaps by reducing anxiety by learning some relaxation. Learning mindfulness. And practicing some skills in controlling your speech, by taking regular breaths, taking a pause between sentences, and trying to maintain a calm manner.