The trailer for the documentary Scott Walker: 30 Century Man makes it look like a mystery: why did Scott Walker stop making records at the height of his fame? Watch the trailer to see for yourself:
But that's only a small part of the movie. Much of the second half shows Walker's output since 1984, which is a slow procession of progressively more avant-garde music. You know, the type where percussionists punch a side of beef. Where a producer says, "Much of the last year has been taken up looking for the perfect donkey sound."
While I'm no fan of this music, or of this type of music, many people are, and this can be seen by the film's interviews, which have a decidedly English focus: David Bowie (who is also executive producer of the film), Sting, Radiohead, Brian Eno, Jarvis Cocker, Johnny Marr, and others lavish praise on Walker and his music.
As a film it's mediocre. Most of the visuals are talking heads (in one montage, it shows his famous fans' faces while they listen to Walker's records), including many interviews with Walker himself in the modern day, which was a big surprise because, again, this movie was supposed to be about a mysterious recluse. The film also includes much of his music. The early clips show Walker as a young crooning mod, but the visuals accompanying the later experimental songs look like the visualization function on Windows Media Player.
I didn't know about Walker before the film, but I have a pretty good grasp of his career now, so in that sense, the film is a success. However, unlike the way I felt after watching Who is Harry Nilsson (And Why Is Everybody Talkin' About Him)?, I can't say I'm a fan now of either his early pop output or his later atonal output. But I do feel a little more respect for the modern "noise" music purveyed by the likes of Eno.