I've always wondered about yukkuri nests like these. Lets say these 2 parents are a bit bigger than a basketball. Where exactly in nature would you find an abundance of locations that can offer the space that these yukkuris + koyukkuris currently occupy? Most tree trunks are not THIS big, and dead trees generally don't hollow out all by themselves anyway. Finding a small cave that isn't occupied would be pretty difficult too, and thus would be prime contested locations, open to invasion by other yukkuri and animals. The only other solution is to dig a hole under some dead tree, which would be extremely difficult for a yukkuri to say the least.
Some time ago I saw a documentary on Discovery, where a frog would dig a trench to move its offspring from one puddle to another. Now, yukkuri can be super dumb by human standards, and many of their habits are suicidal, but they're still light years beyond most animals. They use tools, they prepare for winter, and not because they have it hard coded to get fat in the autumn, but because they know they'll need food to get through the cold months. Etc.
If a frog can dig trenches, the yukkuri should be well capable of digging a hole.
Not to mention that their "claiming" of homes might be displaying behavior that actually works in the wild, so those small caves might not remain occupied.