Man shot dead by police on Tube

"I saw an Asian guy run onto the train hotly pursued by three plain-clothes police officers. One of them was carrying a black handgun - it looked like an automatic - they pushed him to the floor, bundled on top of him and unloaded five shots into him." " BBC
Former Flying Squad officer, John O'Connor, told BBC News "Officers would be trained to aim for the head as shots to the torso could trigger a hidden explosive device."

However why all four bombs went wrong? DR I mean...