Yes, I wish I knew what did it.
He was heading into the burrow, which in itself is NOT like him, the doe was the one who used to get in there.
Perhaps he knew he was unwell and was going in there to rest in peace.
He has never been unwell, he has never tried to scratch or bite me even though he didnt enjoy being picked up.
When children would come to visit, they always spotted him and wanted to pat him so I would pick him up so they could feel his soft fur.
He gave lots of joy.