Surely it's standard police procedure to assume the worst scenario initially and then to downgrade the investigation later if they've got it wrong to start with, isn't it?
For example, when I worked on the railways every incident of someone being hit by a train was always initially regarded as a crime scene until proved (or, at least, strongly indicated) to be otherwise.
Similarly, when the old lady across the road from me was found dead at the foot of her stairs, the street was buzzing with coppers making door-to-door enquiries (on the assumption that they might be dealing with a murder) until it was established that she'd simply had a stroke and fallen down the stairs (or the other way round).
So, until the Leytonstone incident is found not to have been an act of terrorism, the police must naturally start from the assumption that it was.