Poor maintaince, sabotage, bad pilots, bad construction etc. I think it's too early to even hint at any cause. Most likely the oil strips will provide a location clue that can be followed up with sonar and radio localization. The black boxes sends a beacon signal for one month (asfair). But that will be tough to find in conductive seawater.
Once the black boxes (loggers) are found. It will likely provide clues to a new level.
