The centre button separated from the helmet and twatted someone, killing them.Conservation of momentum. The explosion 'pushes' out in all directions equally (since the total momentum change must be zero). Explosive force upwards pushes up on all parts of the helmet equally, hence in theory it should fly straight upwards.
Of course, if your tin lid is not flat on the ground (one edge balanced on a stone etc.) high pressure gas can escape from one edge, the force is not evenly applied and it goes off at an angle.
That's the physics side anyway, Dingerr might have a better ATO version.
You can apply all the physics and chemistry to explosives that you like, but the only important thing anyone needs to remember is that they’re a right bastard.