In most cases mobilephone blasts are happening due to continuous over heating of the device. There are many possibilities to this overheating.
1) Exposing to sunlight or any heat source for a prolonged time.
2) Faulty or duplicate battery getting shorted.
3) Shorting of cutoff coil. There is a coil that acts as a relay when the phone is in idle or switched off. The damage to this coil may result in serious heating issues which may cause an explosion.
4) Keeping a phone in wifi tethering to use mobile data along with other devices while the device is plugged-in to the charger for a long time. This simultaneous charging and tethering will lead to overheating and thereby damaging other components too.
Mobilephones are not designed to use as a WiFi router or tethering machine. These features are there to use only for a short time.
5) Water damage is the another big problem for overheating of mobile phones. Waterlogged phones will get damaged by shorting of various components including battery and it may result in an explosion.
The best way to reduce this risk is to get proper service to your device if it gets a sign of overheating.