For a long time, and possibly still today, Delta used a Hopscotch boarding order. Namely, some in the back, then some in the front, then some in the middle, then more in the back etc.
For example zones (front) 5 3 6 4 2 (back)
(Zone 1 is elite who might sit anywhere)
This did mean that one could assure early boarding by selecting seats in the extreme rear if possible.
The puirpose was to speed things up by have two areas of concentrated stowing compared with just one using the straight back to front method. Like lighting a fireworks sparkler in the middle as well as at the end and it finished sooner. In practice, with back-front boarding the stowing progressed to the front faster than the seating so the frontmost passengers had to gate check their carryons.