Why does my trunk open on it's own when I am driving.

What is your question?

Sometimes when I am driving, usually city-lower speed and when it is raining, the trunk of my car opens on its own. I stop and close it. It will then usually happen again a few times. Then it won't do it for a while so I forget it. Then it happens again, but never when I am at the dealer. The dealership here is terrible and no one has been able to tell me why this happens.