Descending spiral, I’m thinking it seems to be a classic symptom of spatial disorientation.
Or a loss of lift in the wings. Microbursts make it impossible to get any lift, so even if you don’t see a thunderhead, these little monsters can set you down in a heartbeat.
The article did not state if he was IFR rated and current for IFR flight. I remember a flight once from Corpus Christi to Thibodeaux Louisiana. I was in "the soup" all the way. Nothing extraordinary I had a full IFR panel but unfortunately not an auto pilot. I had to fly the whole damn thing. If this pilot did not have an IFR rating and currency I would suspect spatial disorientation and then the classic spiral dive to destruction. Sometimes the wings come off first. Believe your instrument panel but also check the vacuum pressure first. You may be lied to. With rare exceptions most Barons have a full instrument panel.