Possible Causes?
The birdstrike possibility has been discussed locally. It has been a particularly wet Spring this year, and there is open water still extending into paddocks 30nm inland from the south coast of WA. I have never seen so many large birds in flocks up to 50-60 - been here 20 years. They are particularly active up to about 0900 and then again after 1700 local. As I'm up nearly every day instructing, most of which is below 2000' agl, I've had many near misses. I sought a NOTAM with local YABA management on birds last month and it's probably still current.
The witnesses report a loud 'bang' before observing the aircraft in a steep descent. If it had been an actual (multiple?) birdstrike - then there would surely have been carcasses spread about the immediate area? If the breakup resulted from vigorous control inputs, (above Va)..... then what caused the loud noise? Was it the actual airframe breakage, or was it something else, eg, a propeller blade being thrown? The witness reports of unusual, high pitched engine noise might point toward this.
The Va discussion is probably important. The 210B has the same as a 182, (112 KIAS), whereas the strutless models from G onwards, had a Va of 118. Note, these numbers are based on MTOW, and it's unlikely this 210 was anywhere near that - so Va could have been down closer to 100-105. It was possibly cruising at 130-140, which makes the possibility of the control inputs, (during avoidance manoeuvring) directly leading to the airframe breakup.
This 210 accident is one with so many possible causes. And, it is confounded somewhat by the fact of a particularly high time aggie as PIC.