The aviation website Leeham news has posted a list of the standard possible causes that will be investigated in an incident of this nature. They stress that these are not specific to this case, but include:
• Catastrophic structural failure
• Dual engine flame-out
• Clear air turbulence
• Human intervention, such as penetration of the cockpit or a bomb
Seems unlikely to me. There's not a lot of tensions around that and shooting down a flight at 35000 ft would pretty much require a nation to be behind it.
• Catastrophic structural failure
• Dual engine flame-out
• Clear air turbulence
• Human intervention, such as penetration of the cockpit or a bomb
• Accidental shooting-down
• Suicide of the pilot