You're brave keeping them in a cardboard box. My wife would never let me hear the end of it if they got out.
If this is something you do often I would invest in the proper equipment to do it. Just a little cage and at least a water bottle. That will make the entire process better for you and them.
Something like this is what I'm thinking.
OMEM Portable Reptile Terrarium Habitat for Mini Pet Houses https://www.amazon.com/dp/B07H7C5H97...36AGM3CV?psc=1
Exo Terra Faunarium, Plastic Reptile Terrarium, Mini https://www.amazon.com/dp/B000256DXG...605ZAWBQ?psc=1
And this would hang easy to give water without a mess.
Lixit Aquarium Cage Climbing Resistant Water Bottles for Rats, Hamsters Gerbils, Mice and Other Small Animals https://www.amazon.com/dp/B0006B6804...RT8PEQV4?psc=1
As to why it happened? You got me. I had a rabbit growing up that every time she had babies she would kill them and eat the top of their heads.