I just want to point out that it's not just personality disorders that can cause this. There are very real physiological problems that can surface this way, too. For example, some people have adrenaline issues where seemingly minor, or even pleasant things (like running into an old friend at the grocery store), cause a much larger than normal spike of adrenaline. If the person it happens to isn't aware of what's going on, they can react the way their body is telling them to (fight, flight, or freeze). Sometimes if they're aware of it, they can have enough sense to take a pause, but it is often very difficult because the biological response your body has is so overwhelming. It's as unpleasant for them as it is for the person who has to deal with their overreaction to the situation.