It is also possible that cells explode because the venting doesn't work due to poor soldering. If there is too much solder on the cell it cannot vent as it should and then pressure starts to build up causing serious damage or even exploding cells.
Always make sure that you use no more solder than the battery bar is wide so the cell can vent when it has to. To prevent this from happening, use slimmer batterybars. And offcoarse a good high power soldering iron that can do the job quickly and secure.
Joost.