Lithium batteries have a very high energy density. When that's released all at once with a short circuit or very high current draw resulting in thermal runaway, that's when these fires start. The great news is, they're self fuelling fires too!

But, most reputable manufacturers, create charging/protection circuits that protect the batteries against such situations. Making them far less likely (but still possible) to happen.

The problem you're going to get is when there's disreputable companies, operating in countries with less stringent safety laws that are operating the production, processing and shipping entirely outside of the sight of countries with safety rules. Well, then you get a product with a fake FCC/CE sticker on it, that is very dangerous indeed.

I will not buy electronics from those sites for this very reason. Batteries, chargers and power supplies are usually very shoddy from these companies.

It's not to say don't buy stuff made by country X. Because there's plenty of stuff I have bought made in, these countries but sold by companies that DO make sure there's some testing done, and they're not fake stickering everything. But, we all know the companies I'm talking about I think. Also, ebay (because private sellers buy in bulk from these places and then resell them) is something to be careful of too.

It can also be due to unsafe charging (over-voltage) or unsafe discharging (over-current, generating too much heat). The actual fire doesn’t necessarily happen immediately during charging/discharging.