Age

• A battery that has sat in the same position for a very long time can succumb to a variety of age-related problems, some of which could cause it to explode. For example, being in prolonged electrical contact with a metal surface (as found inside a battery compartment) could result in a damaging, corrosive buildup on the surface of the battery. If this persists over a significant period of time, the acid could eat through the protective lining of the battery and cause a violent rupture.
  
  

Impact Damage

• Batteries having impact damage from being used in roughly-handled equipment or from having been dropped, have a much higher chance of self-destructing than non-damaged batteries. When a battery's casing becomes dented, the interior of the battery is no longer functioning correctly. It may continue to produce electricity normally, but the flow of acidic and basic liquids inside the battery has been altered, and can result in the battery being destabilized from within and eventually rupturing.
  
  

Over Charging

• In the case of rechargeable batteries, over-charging the battery results in strained electrolytic fluid levels, increasing the heat of the battery over time. If the battery overheats, the liquids inside might evaporate into gas, causing the battery to burst apart at the seams, sometimes causing damage to the equipment in which it is housed.
  
  

Explosive Gas Buildup

• Due to their chemical nature, batteries produce low levels of explosive gases during their normal operation. Most of these gases are filtered out and neutralized simply through dissipation, and pose no threat. While this isn't usually a problem, if produced regularly in a confined area, the gases can build up enough that they only require a spark or some kind of flame to ignite, leading to a damaging explosion that will almost certainly destroy your battery from within.
  
  

Dramatic Tempurature Changes

• If your batteries are exposed to regular, dramatic changes in temperature, the metal of the battery's casing can become weakened by constant expansion and contraction, allowing for penetration by the acidic internal liquids. If allowed to continue, the battery's case will eventually become so weak as to allow for a destructive case failure, or explosion.