Had a feeling he would have some repressed trauma, wasn't expecting an exact falling injury—I was guessing him witnessing her doing violent act against a childhood bully, the cat .etc.
Daaaaaaamn. I know it was implied she had done something to him, but I didn't think it would be... that bad. Also, what's up with the husband? He's a freaking enabler! No way he wasn't aware of what was going on. And then, in recent chapters, when they were living in that trashed house, rather than helping his son, he just gets all butthurt and leaves.
Parents of the year, indeed.
@BCS I think it's possible due to his small size. Rather than a straight cliff, that place looks more like a slope. Maybe trees and bushes slowed down his fall and prevented any lasting damage?
I think this was foreshadowed before when someone (the aunt maybe?) said that mom had done some to Sei when he was little. This is very similar to what happened with the stranging though, so it's not all that surprising IMO. Seiichi probably suppressed the memories due to trauma.
@Lisahey
The father is worse than an enabler, he's part of the problem, he's known about Seiko's mental state for years and done nothing. She straight-up talked to him in a completely suicidal tone, told him she couldn't bear her life anymore, etc. and he didn't give a flying shit, and now he has the nerve of acting surprised and cry every minute. No wonder both Seiko and Seiichi hate him, the fact that when she wants to be really hurtful she compares him with his father and Seiichi considers it the worse offense is pretty telling.
This is next level messed up. The transition to flashback is so smooth though, making it a good revelation... dang, this is so short but still keep me on the edge.