I'd rather not speculate as to the cause, but contributing factor could have been time in type. Looks like he registered this plane just this month. Very sad situation.
You can't help but wonder if they would have survived without the fire. The now infamous USA Today article spent a lot of time covering that issue. I can't disagree that plane crashes would be more survivable if we could solve the post-crash fire. It may not have helped here, but who can tell for sure?
Diesel recip engines would go a very long way to reducing post accident fires.