Lithium batteries explode, essentially because of material failure... the membrane that keeps the reactants sealed off from the outside air and the electrodes from each other. If any of these membranes are perforated or sufficiently degraded from use, you can have a fire/explosion. I suppose if you use the same battery forever and ever, you could have an accident with it, using it properly but most lithium batteries outlive the electronics they are installed in so that tends not to happen. Only people like me who keep their phones for 10 years because they didn't drop them and don't care about flappy birds v21, are at risk of batteries failing for no apparent reason. Otherwise it's abuse or negligence that causes problems, as indicated above.