Rabbit losing it's hair?

Mites? Some sort of skin issue would be my guess.

It's likely that one of them is a brand new mama. I've had some of my does pull enough fur to line their nests that they looked sadder than a mangy dog. Rabbits line their nests with fur they 'usually' pull from their bellies (so the kits can get to the nipples and keep warm), but some does really go to extremes. I had one first time mother pull so much it looked like I gave her a mohawk ~ the rest of her body had only little wispy patches of fur left.

They can get mites and some other pesty things, but generally if you see one missing a lot of fur ~ especially from the belly and chest ~ it's because she built a nest.