"Out of Nowhere"

When seemingly unprovoked aggressive behavior happens "out of nowhere"...
Dogs are ALWAYS communicating, what are we missing?

References for Body Language of Dogs:

https://www.amazon.com/Talking-Terms-Dogs-Calming-Signals/dp/1929242360

https://www.amazon.com/Canine-Body-Language-Photographic-Interpreting/dp/1929242352/ref=pd_lpo_1?pd_rd_i=1929242352&psc=1