I find it rather interesting that everyone knows that Aizawa died, but the cause of death was never actually disclosed. Then again, when Page 9 of Chapter 1 says, "The reason she died wasn't in any of the news," maybe it's talking about whether her death was an accident, suicide, or murder.
It's also possible that Aizawa herself doesn't know why she died or who was responsible. Remember that in Coco, Hector didn't realize why he died or who was responsible until midway through the movie, decades after his death. Of course, she might have a clue that the living don't know about, one that can help solve the mystery even if she doesn't have enough information or sleuthing skills to do it herself.