Some developers I used to support in the past mentioned that, at this time of year, many devs take vacations to spend time with their families. Because of this, they set aside a week, or even less, from their work time to deliver something to their clients or supporters. Usually, it's standalone content that doesn't affect the main storyline, so there's no need to worry about variables or decision-making. This way, they can take a week off from work without feeling guilty for taking a break.
And many of these renderings used in these episodes were already made throughout the year as standalone content for higher-tier supporters.