Mines and bombs can be complex objects, especially bombs that are intended to not immediately explode and thus deny access | use up human resources due to the poised threat.
Even as far back as WWII bombs from aircraft included booby trapped "anti personnel" bombs with secondary fuses hidden back to kill bomb disposal experts.
In the case of your concern, bombs can be released with "chemical burn safeties" (for example - when released from an aircraft two agents are released that start a timed burn that arms the bomb after X minutes (and the ground is reached).
Other technicques can use the impact force to arm a mine, it's not dangerous until after it's been slammed into the ground, etc.
Short version, the person working with the bomb would call out what they saw, then do what the partner said. The partner had a book and was out of the blast radius. They recorded it all, and if the bomb blew up would write down not to do THAT again!
Eventually they learned to somewhat reliably disarm the bombs by this type of trial and error. And then the other side would add better booby traps to stop them from succeeding.
Even as far back as WWII bombs from aircraft included booby trapped "anti personnel" bombs with secondary fuses hidden back to kill bomb disposal experts.
In the case of your concern, bombs can be released with "chemical burn safeties" (for example - when released from an aircraft two agents are released that start a timed burn that arms the bomb after X minutes (and the ground is reached).
Other technicques can use the impact force to arm a mine, it's not dangerous until after it's been slammed into the ground, etc.