E-cigs and "hover boards": a large number of them are improperly designed, lacking over-charge protection circuits to one or more battery cells.
So even without any modifications, the batteries are being damaged each time they are left on the chargers. Many also lack suitable over-discharge protection, causing the the cell's anode and cathode to breakdown after each use, eventually leading to a short circuit.
Phones, laptops, and POS devices, usually one or more of:
- Previous physical / impact damage due to dropping, crushing (e.g., wedged on reclining seats), excessive vibrations, etc.
- Counterfeit replacement or "extended" batteries
- Manufacturing defects -- e.g., on older laptops with recalled batteries
The rest tend to be loose spares or improperly packaged batteries shorting on metal cases, coins in bags/pockets, etc.