Modern smartphones, no matter how technologically advanced, are all just machines and can react to external conditions like water, heat, magnetic fields and many other conditions. However, a major cause of most smartphones catching fire or simply exploding are faulty chargers or chargers which are not compatible with certain devices.
As has been reported so far, the Galaxy Note 7 explosion took place when it was being charged using a MicroUSB to USB Type C converter. While it may or may not be the sole reason why the explosion happened, it cannot be ruled out altogether.
There's a reason why all companies ship branded phone chargers in the box. There are certified as risk-free and are completely compatible with devices they are shipped with. For the same reason, all phone makers, safety authorities and research firms ask phone users to stay away from third party charging cables.
In December of last year, hundreds of Hoverboards caught fire in the UK due to their batteries blowing up for no reason or speeding up suddenly and making users suffer injuries in the process. The NTS seized as many as 15,000 hoverboards at British ports following safety checks. A number of high street retailers including Halfords and Costco also initiated product recalls and many urged buyers to check if their hoverboards' pin-plugs had BS1363 printed on them.
Cheap Lithium-ion batteries, plugs without fuses, chargers and cabling were chief causes of concern with imported hoverboards which were being manufactured and exported by a large number of Chinese factories. In a positive move, the Retail Ombudsman indicated that retail stores which continued to sell unsafe hoverboards were to be held liable for any injuries thus caused.