Airlines have procedures to board from rear to front. They do this, they say, because it allows faster boarding. This makes intuitive sense, though I have never personally tested it.
The alternative is the “all rows, all seats” call with everyone boarding in random order.
With that understanding, why do airline gate agents consistently call “all rows, all seats” when the plane is running late and they are trying to make up time?
Posted from seat 12F, Delta 1510, 15 minutes after scheduled departure.