It's a huge Tech update that metastasized into a full rework. To make a very long story short, they discovered during the process of updating the backgrounds that a great number of art assets were corrupted and/or missing and had to be re-exported. This in turn broke the rest of the game and necessitated having to re-do everything from posing to code. DC has also been taking the opportunity to add in a bunch of new stuff, possibly in an attempt to make up for the extended dev time.
Username7 has done a great job of chronicling dev comments from Picarto streams and the internal task tracker, so I'd recommend checking out his posts if you want more detail. And
Hero1 periodically posts the official Patreon updates that fill in some of the other gaps. Otherwise, all we have to offer here is speculation.