I'm going from memory here, but I believe the preliminary NTSB finding was metal fatigue of a type peculiar to water bombers, which suddenly lose thousands of pounds of weight with each drop, combined with the stresses of the abrupt maneuvers they have to make to avoid becoming part of the mountainside. This is one reason the majority of the multi-engine water bombers were off-line for much of the last fire season, to do detailed inspections and to revamp maintenance procedures to spot this type of fatigue before it results in another needless tragedy.