Yeah, this isn't a new problem. And it's...probably on Amazon's end? But, it's hard to say. It's not just Image, either. Other comic publishers get hit with random digital delays, too, from time to time.
reddit.com/r/Comixology/comments/1g4yf2u/whats_up_with_amazons_release_schedule_for/
reddit.com/r/ImageComics/comments/1noxc11/recently_so_many_amazon_kindle_comics_delayedwhy/
The Question, Power Company, Last Halloween, Worlds Finest and Detective Comics Annuals have all been delayed a week digitally
bleedingcool.com
I don't think anyone's said anything officially. But, the problem is could be caused by a few things happening. Purely speculating, mind.
One, Amazon does not consider digital comics a high priority. They probably have a skeleton crew working on that department. So, if anything gets messed up, it tends to get lost in the shuffle.
Two, Image might be sending the files to Amazon late, because they don't want any spoilers leaking out. But, when they do that, Amazon doesn't prioritize processing those files. So, they end up getting delayed.
Three, Image might be trying to help brick and mortar comic shops, by giving them "early access" to the books. I don't think this is intentional. But, it is possible.
So, there's a few things it could be. But, I don't think anyone outside of the publishers and Amazon actually know. Could just be as simple as file formats not quite matching up, for whatever reason. Or just simple human errors, like a intern forgetting to send the right e-mail, or something.