Nasty!
Formation aerobatics would have to one of the most unforgiving of pastimes for mistakes. These sort of mistakes can happen for a number of reasons - fatigue, not enough practice, simple brain fart at the wrong moment, whatever. Even pilots in the top formation aerobatic teams have experienced brain farts at the worst moment with fatal results. Unforgiving as I said.
I've watched this a few times to try to see what when wrong. It's my opinion the "inverted" pilot did not use enough forward stick when rolling back to upright. Had he used more forward stick, whilst rolling he would have "pushed" away from the upright aeroplane. It's a very simple and easy to make mistake that can happen when the brain is running at max concentration. It's so easy to relax that forward stick, as negative G is not as "natural" a feel as is positive G.
This brought back memories of the awful Skydancers crash off Portsea back in 1988 with one fatality. This rocked the local aerobatic community at the time. For those interested to read about this - here's the link below.
http://www.atsb.gov.au/publications/investigation_reports/1988/aair/aair198801381.aspxThe ditching was interesting in that pretty well 100% of the time, when a tail dragger lands on water it will flip. The pilot did well and was lucky having people on the beach to help him out. I always fly with a tool to break the canopy in a emergency for this very reason. Even in a span can, doors can get jammed if not opened prior to impact.
RIP the pilot and thoughts go out to his family and friends.
Adam