Alright so...
1) Yes we still have a lot of unexposed ordinance in the ground and we find every year bombs and ammo, but there are very very very few accidents that happen, most of them when construction workers are not careful and simply carry the explosive ordinance away.
Last year we had three die by an explosion.
Now in regards to this bomb, it was one of these nice once that you yanks left us for a little fuck you, its actually really hard to disarm them because their firing mechanism is based on chemicals, and is designed to explode hours after it has dropped and the raid is over to spread further fear and terror and to hinder rescue attempts.
thats the reason they had to detonate rather disarm.