The aerodynamics of an open top box at speed

[Cormanus]

It’s good that no one has answered your question about the aerodynamics, Gone, as it allows me to join yet another queue. I can’t help either. Like you, I find it curious. You’d expect it to slam shut.

I’ve two theories: first, it’s another application of whatever law of nature it is that says things that will embarrass or inconvenience you will occur in spite of all logical expectations to the contrary. The second is that it must have to do with the airflow around the the bike. You could take everything out, leave the lid unlatched and go for a ride with a mate astern of you to see whether it happens again. You may find you have to put something important in there so you can lose it to repeat the result.