Steve Reich has always had an interest in composing music from patterns or processes, and a technique he refers to as phasing (see below).
Early statement (1987) Music as a gradual process.
"Though I may have the pleasure of discovering musical processes and composing the
musical material to run through them, once the process is set up and loaded it runs by
This review of Reich's 'Writings about Music' touches on this also :
"... a new relationship between material and form; not merely the repetition of musical patterns, but the psychology of listening to such repetition; not merely processes of gradual change, but music in which progressive transformation has become the syntax of the language."
Assmbling a piece from a set of repeated patterns is common with him, for example
Drumming. The notes on the YT entry for this piece also touch on this : "The piece employs Reich's trademark technique of phasing. Phasing is achieved when two players, or one player and a recording, are playing a single repeated pattern in unison, usually on the same kind of instrument. One player changes tempo slightly, while the other remains constant, and eventually the two players are one or several beats out of sync with each other. They may either stay there, or phase further, depending on the piece."
On the way that this applies to "Music for 18 musicians" : Composer's notes to Boosey and Hawkes edition.
"The structure of Music for 18 Musicians is based on a cycle of eleven chords played at the very beginning of the piece and repeated at the end. All the instruments and voices play or sing the pulsating notes with each chord. Instruments like the strings which to not have to breath nevertheless follow the rise and fall of the breath by following the breathing patterns of the bass clarinet. Each chord is held for the duration of two breaths, and the next chord is gradually introduced, and so on, until all eleven are played and the ensemble returns to the first chord"