This page takes a scientific approach to explain how creepers manage to survive as a species and reproduce, despite their somewhat counter-intuitive strategy of blowing themselves up as an offensive tactic. I am totally aware that they explode only because it's a video game, and it doesn't need to make sense. This does not prevent us from generating theories as if it were a natural occurrence.
Keep in mind this theory does NOT try to explain why they explode in the first place. Natural selection producing suicide bombing as a survival tactic is extremely unlikely, and an explanation for this would require its own theory. (Yes, I am aware that certain species of real life ant explode, but that's a whole other ball game.)
THE SPORE THEORY.
Creepers can reproduce via spore. They distribute these spores by - guess what? - EXPLODING! Their explosion spreads their spores outwards in all directions. Basically, every time a creeper explodes, there are actually more creepers in the world then there were previously. Well, a potential.
If the spores want to grow into full size creepers, they must land on a host. THIS is why they explode as an offensive tactic. They explode in the proximity of a predator, so that not only will a potential threat to their young be extinguished, but the carcass of said threat can provide food for the spores, enabling them to grow into creepers.
However, this process only works if the explosion kills the target, for the spores cannot feed off a living creature. Evolution has yet to overcome this oversight (similar to their tendency to walk off cliffs), but it still works. After all, a single explosion can produce several hundred-thousand spores, and they can be carried in the wind quite a ways from the initial explosion site (if their is no wind, the shockwave will carry them too).
TL;DR: They explode near an animal and kill it, the explosion spreads their spores (floating creeper babies), and the spores feed on what the creeper just killed.
This theory attempts to explain their consistent lack of hesitation when detonating. It is entirely possible that they reproduce by other, more effective means, such as sexual reproduction (spores are asexual). If this were the case, then the spore method could be considered a "backup". Basically, they would usually reproduce by other means, but if they explode they will still reproduce.
What means they use to reproduce otherwise are up for debate.
Useful posts by other users will be listed;
Your theory doesn't really make sense, because then creepers would never reproduce except when threatened by predators, which would be a really bad way to go about it, plus the chances of the spores getting destroyed in the explosion would be too great (because of the kinetic energy involved).
This is true, but I don't really like the idea of creepers coming from a hive. That's just me though.