I think it's important to emphasize that Kageyama canonically never knew how to interact with people outside of volleyball and didn't have any friends until Karasuno. He doesn't operates like kids his age, doesn't understand the concept of having other interests like videogames, etc. That's why I think basing his relationship with Kindaichi and Kunimi on the idea that they were friends is doomed to make you take sides. Because of that, I'm just gonna be using informations we know are true and a little assumption of mine.
There's a very clear shift in Kageyama's personality. He went from being an wide-eyed, quiet kid who was always excited to play volleyball... To an angry, demanding 14yo who doesn't even look like he's having fun but rather playing with the goal of going pro in the future. He yells at people, is dismissive of their limits and would be at the brink of getting benched if only he wasn't so good at the game. I think the question is less "how did nobody notice?" but rather "why did nobody notice?", and I believe (and this is my assumption) that Kazuyo passed away during Kageyama's second year and the change of attitude happened gradually, rather than immediately. Because that's just not how grief works; it extends forever and it has different stages.
So I would say around his second year he became quieter, more apathetic. Volleyball felt empty now that his grandpa was gone, because that was his coach, his best friend and the only one who understood Kageyama's drive. The change wasn't immediately noticable because he was still playing normally, and maybe he was practicing more and more, but that's just how "geniuses" are right? Maybe he's asking to practice spiking until his teammates are begging to stop and calling him crazy. That's just how intense Kageyama is, right?
When third year rolls around, he's gradually more impatient with -- what he sees as -- the lack of progress on the team. He's more demanding now, sending faster and faster tosses and demanding that the others adjusted to it, because that's how you get away from blockers. You simply don't give them a chance to catch up. But, obviously, that doesn't sit right with the others. They can't keep up, not because they're bad or anything, but because surely no one would be able to? And Kageyama grows angrier because he thinks his teammates aren't trying hard enough, they just need to jump higher and get there faster. Kindaichi and Kunimi just happened to be the most affected because 1) they're the same age and are probably paired together most often, and 2) they're spikers.
So, it's nothing personal. Big deal. They only take it personally because he's being an asshole about it. If only Kageyama was better at translating his thoughts, but at this point in the story, unfortunately he's not. As for the coach... He's just a bad coach! Any person who works with kids should know that you are responsible for them, and your job sometimes goes beyond just teaching. You have to be ready for situations like these, in which a child is CLEARLY going through something at home. I don't know how it is in other places, but whenever someone close to you passes away, your parents inform the school so you're given a a few days to grieve. It's not on the law I believe, but it's common practice, so in my mind he should've know Kageyama was grieving...? Idk
Anyway. I don't think it was ever something super personal, but rather a lack of foresight that no one noticed what Kageyama was going through. And he himself didn't notice the damage he was doing to the team overall. The coach is just a dumb bitch, though.