He is trying to communicate with you. If you “obey” him when he bites, he’s being successful and he will most likely repeat the behavior. This is how learning works. Animals and humans try various things to get what they want, and if one method works, this method will be used.

He nudges you for pets -that doesn’t work, so he progresses to nipping/biting. If you pet him in response to a bite, he will learn that biting is the way to get pet. He can’t see the absurdity in asking for pets by biting, because he’s a rabbit.

As for when you’re trying to get him back into his cage, you could perhaps wear gloves for a while, until he’s realized that biting is not going to get him more playtime.

You don’t need to punish him (rabbits don’t respond well to punishment anyway), just teach him that biting won’t lead to him getting his will. You can say “no” in a calm but firm voice, but don’t yell.

This seems more like a communication issue than anything else.

It’s difficult to say why buns start doing sth. Buns get ideas into their heads. This is why you can never trust a bunny around unprotected electrical cords -a bun can go for years without showing the least bit of interest in cords, and then one day -snip.

Do you perceive his biting as aggressive/defensive? Aggression out of the blue can be a sign of pain/discomfort. It as a rule comes with some other symptoms, most commonly lack of appetite/markedly diminished interest in food and treats. The bun’s activity level might drop, the bun become withdrawn and morose etc.

I’d keep an eye to see so he’s acting otherwise all normal, eats well, toilets well, is active (during his active time, most buns lounge a lot during daytime) and takes an interest in his surroundings. If all that seems ok, his new behavior could just be sth new he’s trying out.

ETA: My bunny sometimes nips/bites or box my mother if she stops petting him. He is very close with my mother, he happily climbs into her lap to be pet and doze off. So that type of nipping or biting is not about dislike for the person. It’s more like a firm request.