Elaina has been considering taking a Labyrinth diving pass for quite some time and it wasn't just a sudden decision she made after the battle with a boss, she was already thinking about it for quite some time.
Ever since the labyrinth was discovered, she wanted to explore the labyrinth and see for herself what challenges she can take and what enemies are ready to fight her. Not only that, but she also wanted to understand why a farm was necessary to form a barrier that would block monsters from escaping.
In short, she wanted to discover the little secret the labyrinth was hiding the whole time. And thankfully, the gods didn't seem too against her in getting the permit...well aside from the God of Farming who is worried about her safety.