[AyaSai]

The reason that Manaka's crazy because in the previous war, her father want to obtain the Grail because he thought this is the only way to reach towards to the Root that they had failed to attain for a thousand years and yearned for thousand years more.

But Manaka has a different plan. She believed such an outcome was so boring, as she had already been connected to the Root since birth, so she instead decided to use it for its real purpose of raising the Beast of Revelation rather than silly stories about granting wishes. She gathered sacrifices, a number of girls weeping as they fell into the Grail, and even her own father and sister.

So, in other words. She was connected to the Roots along, but as she grows up with it, she starts losing her Sanity that lead to her Corruption.