Coltrane said (in the Kofsky interview?) that the reason he "pulled back" for a quick minute was that he messed up his favorite mouthpiece and it took him a bit to find another one that gave him what he really needed.
Conspiracy theorists said the the record company pressed him into it because "Chasin The Trane" was upsetting a LOT of people.
Coltrane countered that he always wanted to have a balanced catalogue and the mouthpiece problem seemed like a good time to do it.
That makes sense to me, because his pitch on Ballads is a bit off, like he was using a less than ideal mouthpiece.
By all accounts he was an inveterit mouthpiece geek. It's SO damn easy to work on one just that one mivro_adjustment that fucks it up, and when that happens, game over.