Its not\ entirely arbitrary.
Its kind of like criminal law mens rea, terrorism is defined by the actors mind. If the goal is to use violence to instill fear, its terrorism, if that wasnt their goal, its not. Thus when a terrorist organization states that it was a terrorist attack, they have basically given you the reason for the attack.
Conversely we dont even know who caused the explosion in this case.
What if it was a spurned ex-lover, that wouldnt be terrorism, that would just merely be a homicide.
The distinction between act of war and terrorism, is going to be somewhat arbitrary, because it is subjective, not objective.