I'm not really familiar with luxating patella in rabbits but I did assist in a few treatments with dogs. Do not take what I am to say as treatment or diagnosis of your rabbit. You should call your vet with any questions about the rabbit. Since he's already treated it, he may provide insight over the phone. Don't stop asking questions until you fully understand what is going on with your rabbit. If you don't understand what the vet is saying, ask another way or ask to come in to see him. Don't be afraid to say "I don't understand what you're saying, can you please explain it a different way?" It is vital that tell your vet what you do and don't understand, and don't be embarrassed about it. You would be surprised at how many people nod their heads at whatever the vet says then come back a day or two later because they didn't understand a word that was said to them.
Basically, luxating means it's dislocated and patella means the kneecap. The kneecap is kept in place by cartilage and ligaments. Like Dood says, it can be trauma or genetic. The leg can look crooked because the kneecap is causing the limb to stay extended. This injury could have happened from the rabbit jumping down from something high, or it could be that it inherited weak ligaments and such from its parents. Often, owners don't know how it happens, they simply notice their animal limping one day.
Surgery can be required to fix severe luxating patella but that is not always the case. Many times, with dogs, joint supplement pills or shots are given to help boost the health of the cartilage surrounding the kneecap. Keeping the animal at an appropriate weight, and providing adequate exercise is also very important. We also did hydrotherapy with the dogs (making the dog walk on a treadmill while it is in water up to its neck) but all of these things require strict veterinarian supervision.
Incidentally, I did do a quick google search for "Luxating Patella in Rabbits" and got back a fair amount of results. I'm sure you can find plenty of relevant literature if you make sure you spell it right. However, google is not a replacement for talking with your vet.
Best wishes, hope your bunny makes a swift recovery!