I agree that the loneliness is most likely a factor coming into play. Although he may never had been “clingy” to you when he was indoors, your presence was there, he could smell you, his usual environment, and he had grown accustomed to where he was. Being outdoors is naturally isolating for a single rabbit, so seeing you, a familiar face associated with that utmost familiar indoor environment, is bound to trigger some high arousal like excitement. While it’s great to have a rabbit be so energized for your attention, I would think about what does that mean it’s like for him when you’re not there?
I am also curious about the change from indoor to outdoor. You are correct, that this forum is for indoor rabbits, and there are times when we have been able to help people resolve issues which prevented them from keeping their rabbit indoors.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.