Then you're not putting it on properly. If the straps are X'd properly, you can't pull it off if you tried.
Both times the strap was used.
The first time it fell off, it was full of paper trash (4 days of camping) and we were flying down some roads in British Columbia taking hard turns.
I stopped after about an hour of that, and it was gone.
2nd time in pismo, I used the strap, and reinforced it with a zip tie. It was mostly trash, and one bundle of fire wood.
After about an hour in the dunes whipping the FJ around I had stopped and it was gone.