... I'm... not actually sure how this happened. There's no sign of any implement or anon or anything that could have caused this... and yukkuri miscarriages generally result in weird, half-formed yukkuris, not melty squashed ones. For that matter, one of them doesn't appear to be fully birthed. Best guess I have is that they were actually already born, then shoved back in and rebirthed.
That’s what happens when you startle a mama yukkuri during a mammalian birth. It clenches its mamumamu and crushes the babies to death. On that note: If your pet yukkuri ever gets mammalianly pregnant without your permission, set up your stereo speakers on either side of its “easy place” and, at the moment of impending birth, turn on Slayer at full volume. Just say “Shit happens!” afterwards.