In the opening sequence of Spectre (2015), James Bond is listening to some bad guys discussing an evil plot. He then starts shooting at them, but they start shooting back - and then, suddenly, the entire building they're in explodes.
It could be that I missed some important dialogue, but there didn't seem to be any readily apparent reason for this explosion. Later on, Bond is reprimanded for it and justifies the destruction of the building on the basis that a greater number of deaths would have resulted otherwise. So the explosion seems to be precipitated by Bond's actions in some way, but that was all I could gather. Did the bad guys have a bomb in the building? It must have been rather a large one. Was it somehow set off by Bond shooting at people? Did I miss the explanation or is it really just not explained at all?
This isn't the only apparently unexplained building explosion in this movie - there's also a rather large one at the end - but that's a different question.