What is it? Well, the following conversation happened on the same chapter, Chapter 29:
Goblin Slayer: You knew everything, didn't you?
Sword Maiden: What do you mean 'everything'?
Goblin Slayer: The cause and the extent of the goblin nest in the sewers. Maybe even who's behind it.
Sword Maiden: Yes...Yes, I did.
Contrary to the other answer, the Sword Maiden did not tell Goblin Slayer's party that they were doing a rat-related quest. Rather, it was implied that they were doing a goblin-slaying quest from the very beginning and that they were already told so before arriving in the water town. From Volume 2 Chapter 2 of the light novel:
Sword Maiden: Goodness. Who might you be...?
Goblin Slayer: We've come to slay the goblins.
If the Sword Maiden knew that and didn't tell the adventurers wouldn't that mean the Sword Maiden risked the adventurers' lives unnecessarily? Isn't that betrayal?
The Sword Maiden, as it was eventually revealed, is traumatized by what happened to her. She still fears the goblins. She said this herself:
After all, if goblins attacked, I’m sure I would just…break down weeping.
Why did she blame the murders on the goblins, when they were just underneath the town? It was revealed that she wants to pin the blame on them because she wants to instill the same fear she had of the goblins to the townspeople. Personally, it would be much simpler if she told Goblin Slayer's party everything since it still concerns goblins and he would not deny the quest.
She probably thought of hiding these details due to the expectations she had of Goblin Slayer. This might be proven by the existence of the ballad about Goblin Slayer, which she heard. Dwarf Shaman revealed to him that 'when word of your doings gets around, everyone will want you to slay their goblins for them!' People would expect someone as strong and capable if his/her deeds are sang and spread as ballads, and Sword Maiden probably expected Goblin Slayer to easily deal with them.
Are the goblins brought by the Sword Maiden? Did the Sword Maiden betray all the adventurers? No, she did not betray them nor bring the goblins. Remember that she was once captured and tortured by goblins. Goblin Slayer noted that the way they (the women) were killed could not be possibly done by goblins. Also, in Volume 2:
Goblins are cowardly, cruel, brutal, and not very smart. It would probably never occur to them to linger in human territory to vivisect and devour their prey. Their unfortunate captives were always taken back to the nest, to be diligently stripped of their virtue there. Or, if the prisoners were numerous enough, the goblins might simply toy with them until they died.
Volume 2 of the light novel explains this properly, I suggest reading it if you have the time. To, again, quote from there:
Sword Maiden was quite aware of the movements of the Evil Sect, against which she herself had once stood. When she had learned of the ghastly rituals of living sacrifice they were performing, she had a good idea of what they wanted to achieve.
Revenge on her.
The goblins were most likely just a tool used by the sect. Aside from revenge, it was also hinted that they plan to do something with the Gate Mirror, albeit it was not specified.
They mentioned something about no rat quests. Why? Is it related? When Sword Maiden asked why Goblin Slayer suspected her to be hiding some information, he told her it was for a number of reasons. One being that he didn't think the encounter with the white alligator was not planned, simply because it chased them away but attacked the goblins, and another being the town having ruins yet there were no maps, rat-killing quests, adventurers nor patrols underground. He said it was impossible and that there had to be something standing guard down there, leading him to the conclusion that the white alligator was a familiar, guarding the ruins.