Thats different - All the times I've experienced our rabbits twitching their bottoms is when they are horny and want to be mated. Who knows, maybe their enjoying there food?
I noticed some of mine do it when they are eating their cecotropes or even something they just really like like a banana chip... Maybe they're just excited?
It's only 2 of my females that do it? I was thinking maybe it meant they were excited because when i bring the food out to them they binky everywhere then twitch their bum when they eat it, well it's not their bum it's like they've got an itch but are too busy eating to scratch it, but they only do it when they eat? lol