Or simply a poor batch of explosives. It seems the bombs were intended to explode in exactly the same manner as the bombs 2 weeks ago, but, whilst to primer charge went off with a small explosion, the maion charge failed to detonate for some reason.
My own thoughts are that they were trying to build up a momentum of attacking one city, the more often they succeed, the more recruitment. However, this may well have backfired on them at a fatal level, since this was a complete cock-up on a military level, it caused some disruption, but we've had a bit of practice at these disruptions before

Better still, I would say it is likely that one or more of the intended suicide bombers will be captured, in a way it is urgent that we do, since these people really have nothing left to live for now.