Mobile phone explosion
As far as I know the main reason for these explosions are that their Lithium-Ion based batteries overheat. These problems are caused by manufacturing defects or sometimes bad design (in the Boeing 787, see here)

The energy does not need to be introduced to the system as it is already there inside the battery. What can happen is that the battery shorts internally discharging itself through that short. This generates a lot of heat which can lead to fire and/or an explosion.

There's an interesting article on this subject here at Batteryuniversity.com