Jackonicko
An AEZ is requested for any incident where a bomb is suspected. Be that terrorist, or WW2 device.
The reason being somewhat obvious...big bang, bits go up. Unless you know exactly what size said supected bomb is, your guess as to how high is as good as anyone elses.
There is an element of keeping the press out of the way as well, especially if there is a police air support unit involved. Do they really want to be dodging the press while trying to do a somewhat more important job? No.
As to your opinions of accident scenes, it's like any other crime scene(yes, thats right, crime scene) and the time it's cordoned off, depends on many things, ranging from how long it takes the specialists to get to the scene, to how complex the investigation is.
Yes, Policing is based on consent, but that has to be balanced against protection of life and property, whihch is the primary object of Police in the UK.
Your recollections may have been based on simple accidents, and nothing complicated.