People are simply oblivious when they are boarding. I also use the aisle whenever possible, and I have been whacked plenty of times by oversized backpacks. When I board a plane, I always either carry my backpack by hand, or I place i on top of my rollerboard. The vast majority of passengers are clueless about the physics of turning around while wearing a backpack. But there are far worse things - lately I have been getting assaulted by oversized butts as pax turn around and bend over to fiddle with their carry-ons resting in their seats. And unlike backpacks, I can't really give their backsides a gentle shove.