What could cause lameness in the back legs of a young rabbit?

My friend has got (as a present for their children) 2 around 7-week-old male (?) rabbits 2 weeks ago (when they were 5 weeks old).

Like I wrote in another question, she has got no information about the food that they have got before.

Now she sent me a video, where one of them could not or only very lightly move its back legs. I send her straight to the vet.

But what could cause this? And maybe, how would it be avoidable?

I know (and told her) that rabbits are very sensitive to stress. These little bunnies now had a lot of this: first time without mother or siblings, moving in a new home and get new food…