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.