Each blocked is hashed twice. Why isn't one application of SHA256 enough?
SHA-256(SHA-256(x)) was proposed by Ferguson and Schneier in their excellent book "Practical Cryptography" (later updated by Ferguson, Schneier, and Kohno and renamed "Cryptography Engineering") as a way to make SHA-256 invulnerable to "length-extension" attack. They called it "SHA-256d".
My conjecture is the double hashing everywhere was a red-herring to make us think Satoshi was sloppy, lame and take our focus away from a posited valid use case for the
My lengthy and elaborate rationale is in my answer on the related question.