From my personal experiences and interactions with people, it seems that some people without Bipolar experience racing thoughts with anxiety in high pressured situations or at night when they are thinking of too many things, as a result of stressors they are experiencing: or not even anxiety, but racing thoughts based on things that happen during the day, like you said, things popping up in the person's head.

What I personally experience with BP, my racing thoughts distract me from carrying out responsibilities, or the thoughts jump and skip around in a disorganized way that interferes with my day or night. I become very speedy from them. Or with anxiety, they sometimes cause me to hear voices at night. During the day and especially night, the racing thoughts get loud in volume and cause me to be very agitated....even confused at times, often very unfocused.