Not every delivery failure is due to poor reputation or spam. Sometimes ISPs just have problems on their mailservers and so mail doesn’t get through. It’s often hard for delivery experts (and their bosses and their customers and their clients) to watch email delays or rejections without being able to do anything about it.
Sometimes, though, there is nothing to do. The rejections are because something broke at the ISP and they have to sort through it. Just this week there’s been a lot of twitter traffic about problems at a major cable company. They are rate limiting senders with very good reputations. They have admitted there is a problem, but they don’t have a fix or an ETA. From what I’ve heard it they’re working with their hardware vendor to fix the problem.
Hardware breaks and backhoes eat fiber. Yes, ISPs should (and all of the large ones do) have backups and redundancies. But those backups and redundancies can’t always handle the firehose worth of mail coming to the ISPs. As a result, the ISPs start rejecting some percentage of mail from everyone. Yahoo even has a specific error message to distinguish between “we’re blocking just you” from “we’re shedding load and temp failing everyone.”