Everyone seems to state that they didn't know how big this explosion was going to be. I strongly doubt that. Ammonium Nitrate is a commonly used explosive so the size of boom is well known.
I had the good fortune of witnessing the disposal of a 39,000 lb solid rocket engine last summer (a considerably larger boom than 500 lbs of ammonium nitrate) and the entire morning the demolition crew was monitoring the atmospheric conditions to guarantee that the shock wave wouldn't be reflected into Salt Lake City. It seems that in a previous disposal the atmospheric conditions were such that the shock wave from the explosion was reflected over 20 miles into the city where it too broke some windows.
This would seem to be a similar case. The size of the explosion would have been well known, however the technicians setting up the explosion may not have been aware of how the atmosphere could propagate the shock wave of a detonation of this scale.
Different air temperatures, or wind directions at the time of the detonation could easily have made this a non-issue.