So there's a mastermind who arranges for things to fall conveniently in place for our party? Well, we already know who that is, it's the author..
It's not an uncommon trope for dungeons to be controlled by a sentient mind, if not be sentient themselves; in fact, I'm re-reading a novel series right now about video game-style dungeons where the dungeon is commonly theorized (and even shown in one book) to be sentient.
It
wants people to dive the dungeon, because it gains sustenance from everything divers lose inside (blood and flesh, obviously, but even skin cells), so it offers rewards for those who overcome its challenges, but also has pretty strict rules about how people actually
handle said challenges; the most common to come up is the "one party" rule, where only a single party is allowed to tackle a given challenge (other than proper raids). Anyone who breaks that incurs the wrath of the dungeon, who either massively buffs the boss or summons a swarm of monsters to drown the rule breakers.
And it's exceptionally common for dungeons in manga to have a dungeon master (hell, I follow
multiple ones from the perspective of said DM) who controls the direction of the dungeon, from difficulty to lethality to rewards to the entire layout.
TL;DR, yes, of course, it's the author, but in universe, there's plenty of explanations for why a dungeon would become so seemingly accommodating, despite being so deadly.