It could be the weight distribution inside your bag or the strap might be a little loose? I use a Timbuk2 and don't have the same problem as you. The last one I

It could be the weight distribution inside your bag or the strap might be a little loose? I use a Timbuk2 and don't have the same problem as you. The last one I bought I learned they're only "water resistant" even though I've taken mine in downpouring rain and everything stayed dry.