for 20 years, I've used a messenger bag when carrying gear on the bike. I've used lots of backpacks when I am in pedestrian mode. I cannot explain it.

I think it has something with center of gravity.

But try this on - a backpack has two straps and requires two hands to adjust it. Think about tightening or loosening the straps - you do them at the same time. A messenger bag, with its one strap, requires one hand to adjust the load; be that side to side or up and down.

But like I said, I cannot explain it.