If the phone was powered down, it looks like this might be a chemical reaction in the batteries. Taking it out might just remove barriers between the source of combustion and flamables in the environment.
First I'm surprised Al Queda doesn't use a special OS or a virus to make phones explode on a plane. Second, it could be competitors who specifically target the Note 7 with a 0-day attack to harm Samsung's reputation.
All phones with a battery are a fire hazard if they get the wrong OS.