Strange agressive behavior

[Shini]

For over 8 months I've been dealing with some very strange I suspect territorial behavior with my BT where outside of his enclosure he is extremely docile and gentle, he can be handled, touched and picked up without so much as a huff of annoyance, but when placed in his enclosure he becomes a completely different beast and extremely aggressive, it's not just defensive he will actively seek me out getting as close to me as possible and lunging trying to bite, this behavior is consistent year round not just during certain seasons. Within seconds of being put back into his enclosure this behavior becomes apparent it's like Jekyll and Hyde it's very odd and alarming.

For some history, he is 15 years old and I have had him since he was 1 hes always been with me.
Confirmed male by vet and the presence of sperm plugs.
Not regularly hand fed,
Regularly handled daily.
He did have an ulcerated eye which was treated but this behavior was exhibited both prior to having the issue and after treatment, vet gave him an all clear so there is no other conditions that may be causing this that are known.
His enclosure is large 70cm x 70cm x 35cm, 170 litres with four places to hide, heating, UV and soaking dish.
His diet is varied with the exception of snails as I cannot find a source I would trust not to be poisoned or have parasites, his diet is supplemented with Vit D and calcium.
As far as my vet and me have observed he is healthy, a good weight, active and has a great temperament(when outstide of his enclosure).