Sudden loss of a loved one is shocking, wanting to know what happened is natural and to try to make sense of pain. Alls can say is, don't know...an extremely good person is gone.
Sadly, thru grief it is a reminder that we are on borrowed time with one another. Significant relationships that have meaning to each of must and should be treated with important care. The world gives us no warning, a reminder of unpredicatability. Do the "wishes", say the i love yous, say the apologies, forgive, give the hugs, treat with kindness and empathy. Don't hold off for tomorrow.