I think it was the anon doing it, and then buried them for some reason. If he'd done it enough times (and it looks like he has), he'd probably know that the ko might typically dig to try and escape, so he put them back for some mental torment. Or something.
Ah, I think the smaller Marisa was old enough to leave the nest, but couldn't get very far because it was living in a tank, so tried to escape by getting on the roof of the tank. This is when anon took the accessory and pushed it back in, making the parents aggressive towards the "uneasy" yukkuri, and it starved to death. Still doesn't explain why the accessories are buried in the corner though.