There are many ways to speculate about this; maybe he ran into some unfortunate malpractice/legal issues, maybe he hurts his fingers or hand, etc. But if you read some his editorials in neurosurgery journals, he was always ardently arguing about how neurosurgery needs to become more competitive before it loses out to orthopedists doing spine, interventional neuroradiologists doing aneurysms and embolizations, neurologists doing NICU and head trauma, pain specialists doing interventional spinal procedures, and so on. Maybe his move into radiology is a show of defeat, or an effort to bring neuro-interventional procedures into the realm of neurosurgery.
Of course, neurologists can also bring interventional into their realm of practice if they work for it.