You will also see stuff getting airborne when a concrete road deck shatters because the supports underneath it have given way.
Two things. First, when a steel reinforced concrete fails catastrophically, there is an incredible amount of stored internal energy to release and so it will appear to explode - because it does. It was strained to the point that it could not take any more - by definition.
Second. If there were emplaced explosives of the quantity required for demolition of this sort, it would immediately be detectable. In fact you could smell it anywhere in the vicinity, without any difficulty whatsoever. Even well placed shaped demolition charges would be easily sniffed out, and bystanders would be under no illusions about what they were smelling.