I'm guessing you are already using the "random time" function of the logic_timer? I don't know how many "a bunch" is, but logic_case can support up to 16 outputs that can be randomized.
All you need to do is put in an OnCase## output for each of your explosions, then have the timer send PickRandom to the logic_case and it will select a random output that has links and fire it. The PickRandomShuffle inputs ensures each individual output is fired once before any are repeated.
If you have more than 16 explosions, you can either chain the logic_case (one case that picks between two other cases, which then pick the explosion), or just have another timer and case.