Portulaca plants are edible for humans, too. So it's not surprising that rabbits find them tasty. Because the rabbits have already discovered the plants, they may keep coming back. Various rabbit repellents may be effective. Liquid Fence, Messina Wildlife Control, and Plant Skyyd, are several brands that are generally effective. If all else fails, you could put the containers up on stands to get them out of reach of the rabbits.