A bad batch of explosives? A rush job? Some speculate that there was great cleverness involved, but it's more likely to have been incompetence:
BBC security correspondent Gordon Corera told BBC Radio 4's Today programme the devices were so similar there was speculation they could have been part of the same batch.It must be a rather peculiar feeling to detonate a suicide bomb -- and live. There is still the unlikely possibility that someone sabotaged the attack.
'The explosive might have degraded over time or had not been put together right in this case, or it could have been a completely different batch of explosives - homemade - that had not been cooked up properly.'
The bombers' plan might have been disrupted by the investigation into the 7 July attacks, forcing them to act before they had been fully prepared, Corera added.
Some witnesses said the Thursday's attackers seemed 'scared' or 'surprised' when their bombs failed to explode properly.