I believe Kyrie's grandfather's passing had a significant impact on him, and I am open to believing that Kyrie believes that that trauma is the reason he left Boston. But I don't believe that was the root cause of why he left. Sorry. Some people convince themselves of things to prevent them from dealing with hard truths, and I think that's what's happened here.
The hard truth is that Kyrie started talking to Nets players, and to Kevin Durant, very early on in the Celtics' 2018-19 season. There was no reporting whatsoever of Kyrie suffering a loss in the family, or even that he was going through personal struggles, in the innumerable stories about his frayed relationships with the coach and teammates.
There is little to no explanation for why Kyrie felt that living in Brooklyn, NY during a portion of the calendar year would be better for his mental health than living in Boston, MA. Keep in mind that this is the same person who, about one year ago when asked about the possibility of him going "back home" to play in New York, responded: "I'm from New Jersey, just to be clear."
The most likely explanation is that Kyrie's team struggled out the gate in the 2018-19 regular season, he saw a long and difficult road ahead with a high probability of failure (as in, the team probably wasn't going to the NBA finals), and he started looking for an escape hatch.
That doesn't making him a bad person, and it doesn't mean he's some sort backstabber or something. That type of language is way too harsh. This is the man's life and he can do what he pleases. But the truth remains that he effectively made a promise to the organization and his teammates and then backtracked quickly thereafter. At least give him credit for acknowledging that he didn't put in the effort that the rest of the team deserved out of him. But that's about it.