IIRC the official reason was that some automated anti-spam code accidentally caught the kbin user agent and mistakenly added it to a block list, and the lemmy.ml admins were busy and didn't see it for over a week - but once one of them noticed it was promptly fixed.

Also, I recall this being specific to lemmy.ml - other instances run by other admins like lemmy.world and lemmy.ca weren't affected.