If I understand the neurology behind Bipolar correctly, our brain chemistry works in a way which leads to 'faster' thinking, which is why we often take medication to slow our brain down (rather than speed it up as in treatment for depression). I can't point you to any studies, but this has rung true my whole life as I've always been a very, very quick thinker.

The problem is, when your mind races and you think more thoughts, the proportion of paranoid thoughts increases, and paranoid thoughts can snowball into an even greater state of paranoia.

Imagine you aren't bipolar and think 3 thoughts a minute and 10% of your thoughts are mildly paranoid. This means that you only have one paranoid thought every 3 minutes. But if your mind is racing constantly that whole time suddenly that 10% paranoid thought rate becomes 1 paranoid thought per minute. Then, you think the wrong thought too often and it more frequently snowballs into greater paranoia.

This is why I tend to stay away from things that make my mind go faster (caffeine), and tend towards things that slow it down (lavender, chamomile, beer), which always leads to much clearer, and normal patterns of thought.