in back-to-front, a line forms as you are waiting for people to store their bag. one person can take up [block] 3 rows of seats while he is fiddling with putting his bag overhead.
in random seating, people all over the plane are storing bags at the same time, instead of just 1 or 2 people.
The most aisle time I've taken on any flight might be 10 seconds, though most of the time it's zero since I prefer to store my carry-on below the seat when I can.
From the simulation it looks like the bottleneck forms because each person takes a minute to put their bag away. So if you have 1 person in the last isle putting their bag away they are basically blocking the entire plane.
With random you get random people who can access the overhead compartments all over the plane which helps increase the number of people simultaneously putting bags away.
The people in the aisle are units of work that need executing. In back to front, only the unit that is right at the front can actually be completed since only she is blocking the people behind her, who are in turn blocking the people behind them.
In a random setup, say the first person goes right to the back of the plane, there's every likelihood that the person behind her is not sitting next to her, which means that 2nd person can immediately reach his seat while she does the same..
Multiply this a good few times, and you'll see that a number of people proportional to the total number of passengers will always be getting seated at one time instead of just 1 or 2..
Then again I avoid airports these days because of the TSA.