Simple explanation. The detonater went off, but the mixture had either been sitting too long, gotten to cold or was mixed with the wrong ratio and lost its composure. Hence there only being a small explosion which blew the ruck sack open but didn´t bring the nasty mixture to go up.
Most bombs have a pen sized detonator which is stuck in or near the explosive depending on which type it is. The explosive cannot just explode on its own it needs help by a smaller explosion.
I have a bit of knowledge gained by my 4 years in the military