Maybe it'll be a part of imae's backstory. I mean, we're not done with their backstory are we? However for now, the reasons are still unclear
When Imae fell into the water, you hear people accusing him of killing a village's god and his "mother" claims he stole everything from her. It's implied by those and Imae's caster circle when he grabs his mom, that his powers activated and he accidentally killed the deity that his mother channeled along with the water spirits. Since his mother is a mystic if the deity is gone, she no longer has her powers and since mystics promise their entire being to serving their god, losing it is probably a big shock.
Wait im kinda confused on something why did imaes mother get mad at him after he saved her? Can someone explain thanks