I used to fly and maintain a C421, and even with a through 50 hr inspection, I still had an exhaust failure. The give away was the MP decreased to ambient. The only thing to do is shut down the affected engine. Even with the quick shutdown there was engine mount head damage and cowling damage. It doesn't take long.Saw on the news a witness saw an engine on fire before it went down. Not sure how credible.
On another note, my boss gave me a letter from Cessna three days ago stating their concern that owners and operators have not been complying with the exhaust AD that plagues the twin Cessnas. We have been compliant, but the memo basically said that some reports from overhaul shops had seen significant increase in aircraft with damaged exhaust. Those aircraft were not keeping up with the 50 hr inspections or the other inspections involved.
This is no speculation on the cause of the crash, just a reminder to be good with MX. AD's are out for a reason.