High school boys don't use their lockers. I can't tell you why, though I've asked my two teens a number of times. Not cool, I imagine. Using your locker means you have to stand by it, where you run the risk of being seen standing by your locker, which must be somehow bad. Thus they carry these enormous backpacks crammed with all their books all day long. The packs must weigh 30 pounds. Attempting to heft one is like trying to lift a fire hydrant.
Thus high schoolers are accustomed to hauling backpacks. So it should not be surprising to see that the backpack fashion has migrated, as fashions do, from the young to the less young, as those who were carrying backpacks in high school five years ago now rely upon them to tote their necessities to work.
The change has been long in coming, a number of years, with online chatter going back to 2012, 2011, about whether backpacks are appropriate in a business setting. That debate is over, settled. This spring has been the time, during the trudge from the train station to work, that I registered that backpacks have definitely won, though I might be influenced by Motorola moving to the Merchandise Mart—they gave their employees company backpacks as a welcoming present, stuffed with corporate policy books and giveaway pens and lyrics to the company song, no doubt. Motorola backpacks galore. Many companies do the same--perhaps in part because backpacks have an aura of priciness but are actually pretty cheap. I see lots of backpacks emblazoned with tech company logos.
Why did backpack prevail over the traditional briefcase? Several reasons, running from the practical to the psychological. On the practical side, we carry fewer papers and books and magazines and, sigh, newspapers and other things that are flat, but more devices and 3D objects. Your backpack is your laptop case, your gym bag, your lunch bag, your shoe bag. If you're carrying heavy stuff around, a backpack more evenly distributed the weight across your shoulders, while a heavy briefcase tends to tilt you to one side.
Then there are the psychological factors. Backpacks are active, young and sporty, while briefcases are sedentary, stodgy and old. We are all climbing Mt. Kilamanjaro now, at least in our own minds, and a backpack implies that you have just stepped out of Estes Park and are making a necessary dash through the Loop before returning to your cabin in Idaho. Many people carry water bottles, jammed into little mesh holders on their bags, to stay hydrated on the hour trek downtown from Naperville. Nor are the bags simple; they are silly with compartments and zippers and flaps and carabiners and handles and straps: complicated bags for complicated people.
At least in our own minds.
As a fuddy-duddy, I was inclined toward briefcases. As a young man I carried one of those big leather legal briefcases that opened wide at the top, the better to jam more books inside. I also liked envelopes--lovely pebbled leather cases without handles, designed to be tucked under your arm. I could never carry a backpack because I'd feel like somebody is coming up from behind, grabbing me by both shoulders, pressing a knee against my back and pulling. Even hiking I prefer a belt pack. I also often wear suits, and carrying a backpack with a suit is like wearing sandals with a suit. It's just wrong. For me. For the moment.
Even so, when it came time to replace my latest briefcase, I had a consideration that was new for me--I wanted something I could fit my Bell bike helmet in.

What's next? If fashion trends are being fed by high school--a safe bet--that means in a few years businesspeople will wear cargo shorts to the office all year round. At least for a while, before they stop going to the office entirely. Nothing to cry about. If fashions didn't change, we'd all still be wearing spats and carrying canes. And if you feel bad about the briefcase vanishing, by all means carry one. Idiosyncrasy is always in fashion.