Mostly, li-ion fires can be divided into 2 causes: impurities introduced during manufacturing, and bad battery management systems.
Unfortunately, batteries seem to be poorly understood by most electrical engineers, so the amount of flawed battery management systems out there is rather huge.
Still, the 'rules' are pretty simple: Don't go below 2.80V, don't go above 4.20V. Ever. Not even for a millisecond.
Our clever cost-aware EE will then think "I've got 3 in series, so that means I need to stay between 9V and 12.6V. This is wrong, he needs to monitor all 3 cells individually. There will never be cells matched exactly enough, and stay matched throughout their service life.
So, each cell must be monitored individually. On discharging, the device must shut off when the weakest cell reaches the minimum allowable voltage, or sooner. On charging, the charging current must be reduced when the fullest cell reaches maximum permissible voltage. Preferably you have a circuit that can bleed off some charge from the fullest cells, so that all cells can be charged full.
The EE that reads the datasheet closely enough, will notice that samples if cells are put through overdischarge and overcharge testing, wherd they are taken outside of the permissible voltage range, and demonstratedly don't explode, smoke, or emit excessive heat. He will then make the assumption that the limits are more of a guideline for best cycle life, and the worst that will happen is that the battery dies a bit sooner, and the customer needs to buy a new battery or device sooner.
While it's true thay li-ion cells must withstand such tests without failure, they only need to do it once. That is, one test is performed once, and the specific cell in question is never used again.
The reason for this is that every deviation outside the safe range inflicts accumulating damage to the cell. Accumulating meaning that while a 'small' deviation from the safe range might not cause problems the first time, it could cause issues the 10th time or the 200th time it happens.
At one extreme voltage, the copper inside the cell starts dissolving. If this process reverses, you get conductive copper in random places, potentially causing short circuits, or a string of thin copper might carry current, cause heating, and ignite the cell.
At the other end, the battery evolves metallic lithiun, which is the equivalent of the petrol in your car turning into nitroglycerine. Ungood.