Unintended Intro - iii-VI-ii-V Descending Four-Note Sequence
It's basically a four bar sequence which repeats with rhythmic, melodic and directional variations the second time around.
The idea here is to maintain the integrity of the four note cell per measure / chord, while connecting the following measure / chord via a descending (mostly) whole or half step.
This results in the overall descending shape of the line.