It's a laptop backpack with straps that can move around a bit. All it takes is someone to hook a toe on a loop, and then that person would go headfirst down the slide.
One of the first things crew does after emergency is to make sure that the exit rows are clear. If your bag does move around during the crash landing and blocks exit, it will be removed out of the way
The point still stands - if it can be tripped over you are not stowing it properly and it should be in the overheads.