I work in a petshop and had a surprise litter, the mum gave birth to 3 babies and was well in herself.
It seemed to be going very well, except she had a tiny little runt, one middle sized and one huge baby. All survived to 3/4 weeks old, then they started to get resporitory problems and I went on holiday. When I came back they said they found one dead then mum killed the other two. Mum was completely healthy and happy with other rats and people, she had no resp problems like most rats in petshops have so I have no idea why this happened.

They were lovely little babies.
Again I know of someone who took in a big agouti rescue, all the rats rescued were agouti and most were pregnant, one of the mums suddenly turned on her 3/4 week old litter, I think killing one before they had to be removed. There was no reason for her to do so.