
(SPOILERS AHEAD)
Not a detailed explanation and might not be 100% accurate as I only heard of this from other people who read the novel raws.
Maxi started learning magic after she found out she has the potential to be a wizard? magician? just like Ruth. Now that she is a wizard, a huge fight/war broke out that caused Maxi to overuse her magic and she almost died in war so Riftan sent Maxi back to Croix castle thinking she would be safer in Croix. Then I believe Riftan tried to go to Croix to see Maxi but the duke didn't allow him to see her. There, the duke was constantly beating Maxi inside the castle and Riftan pretty much barged into the castle and saw what happened. Riftan was really angry so he pushed the duke off the stairs (the duke was really bloody as a result) with the help of Maxi's half-sister to cover up his act and left the castle with Maxi. That's when Maxi told Riftan what she has endured in the castle.

There is no current translations of book 2 and book 1 translations are barely close to finishing but you can read a lot of spoilers from novel updates forum here:
https://forum.novelupdates.com/threads/under-the-oak-tree.97759/
But be mindful most or all of them merely MTL-ed the novel so it might not be 100% accurate.
I've read the translated novel, but it still not that far ahead. Will Riftan ever find out that maxi has been abused?