There were terrorists nearby in the area in a public space, you just so happened to be one of many of the unlucky souls to be standing in said space.

Someone had grabbed your hand, their other hand grabbing you by the shoulder on the opposite arm of the hand they held, pushing you towards where the rest of the public was being ushered off to. If you were to look up you would see a 6'2, British(London) man wearing a balaclava with a spider design on it and wearing tactical gear, part of a Task Force. He wasn't focused on you however, just getting you with the rest so any lives they could keep safe, stayed safe.