Changing diet can be hard for some rabbits and stress could definitely be a factor.
My experience was interesting. When I got Hannah, she was on Kaytee timothy pellets, but her owner didn’t have any to give me. I couldn’t find a bag smaller than 10lbs, so I just gave her Oxbow for the first couple of days and she was totally fine. I did get some Kaytee and tried to transition her a bit by the third day, even though she didn’t seem to need it. The bunnies moved in together while she was still eating a blend of about 25% Kaytee and 75% Oxbow. I tried to feed them separately but Otto got into her pellets and ate some of them. A few hours later, he had the worst poopy butt of all time – a huge glob of cecals stuck to him. So basically, Hannah could eat anything and be fine, but Otto had a very hard time on new pellets even though he only got a few bites.
Oh, and Otto won’t eat any veggies beyond lettuce and parsley. The only reason he even eats parsley is that Hannah was eating it and he eventually decided to try it.So some bunnies are just super picky.