The problem in practice with boarding people in back first is that:
1. They tend to put their carry-on in the first bin they find in the front of the airplane. So people sitting in the front have to put theirs in the back. This is a disaster later in the boarding process because there isn't enough room for people to circulate back up to the front after they stow their carry-on.
2. Frequent fliers and people who purchased premium economy seats expect to be able to bring a carry-on. They tend to sit in the front the plane, so the problem for #1 affects them more.
This is relevant to United's new policy because boarding people with windows in the back board before people with aisles in the front.