The Li-Pol battery explodes after a failure of the protective circuit, which is a standard part of the battery package. The protection circuit is sometimes (rather never, but happens) outside the battery case and is part of the device's printed circuit board.
However, the primary cause of a battery explosion is a failure of the battery cells and a failure of the protection circuit is a secondary problem.
If the protection circuit fails, a failure of the cell would not cause the battery to explode.
Of course, there is also a failure of the battery cell, which the protection circuit cannot secure and the battery explodes, which is a direct structural (manufacturing) defect of the battery cell.
