Order of entering an airplane?

[question everything]

I have not travelled for many years and was used to agents calling travelers to enter by row, starting from the back and with many grabbing the storage compartments in the front.

The other day, at JFK, at Delta, our boarding passes included Cabin 1 and Cabin 2.

The gate agent - a wonderful competent one - told us to form two lines, on both sides of the gate, no possibility of crossover. The first to board were the front rows, and then the back ones. It made so much sense. We had all the overhead compartments available.

So I am curious of whether this is how it is no done.