It's very unlikely that the eye issue was due to broccoli. Food allergies don't usually cause problems like that. More often it's digestive upset, poor skin/coat condition. I also haven't heard of food allergies happening in rabbits. It does happen that some bunnies cannot tolerate a given food, but those usually lead to gas or diarrhea, mostly GI diseases related to the gut flora balance. In addition, allergies should affect both eyes. Your bunny probably scratched his eye somehow, or had something stuck in it. If it comes up again, a vet should be able to help. Infections will occur in one eye at a time, and there are other problems (mostly tooth-related) that will show up as eye problems on one side only. Rabbits have very poor immune systems in general and aren't typically susceptible to autoimmune diseases like allergy. They do happen sometimes, but the only kind of allergy I've seen in a rabbit was due to dusty/moldy hay.