It's actually very simple.
A bakuhatsu 爆発 is just an explosion. The expanding ball of fire and smoke, or even something of smaller scale. It's what bombs do, they "cause explosions", bakuhatsu suru 爆発する, it's what fireworks do, they explode, but nobody says "fireworks cause explosions" unless they are talking about an accident that exploded a building or something.
A bakuretsu 爆裂, on the other hand, is when something explodes and is torn apart or broken apart because of it. Well, of course a bomb is going to bakuretsu if it bakuhatsu's, but it's supposed to, so nobody says the bomb bakuretsu shita 爆裂した. A better example would be someone's castle, which didn't explode by itself, but was torn apart by an explosion.
Now if you ask me why Megumin says bakuretsu and not bakuhatsu in KonoSuba, despite the fact she literally creates explosions, that would be because (according to herself) Bakuretsu Mahou 爆裂魔法 is the strongest type of bakuhatsu-kei 爆発系, "explosion-related," magic in that world. It's just a cooler word, like saying "explosion magic" is tier 1 magic and "detonation magic" is tier 2 magic. Similar meaning, different power levels.
Here are some other related words:
- sakuretsu 炸裂
Burst apart. (balloons)
- bakudan 爆弾
- bakuha 爆破
Destructive explosion. Destroy by exploding. (buildings)
- bakugou 爆轟
Detonation. (an explosion strong enough to cause shock waves)
(namesake of a character in Boku no Hero Academia)
- shougekiha 衝撃波