Roof Vent Opening by Itself?

[Too Tall]

So I just replace a cracked vent. No it opens by itself. I close it, take off and a few miles down the road, it's open approximately 45 degrees. Anybody have this issue? How did you fix it? I'm think the air flow is causing some type of vibration that's causing the vent to open itself. I'm thinking of tie tie wrapping it or alternatively installing a fantastic vent but really don't want to do that.