Only a medical doctor can tell you for sure why your grandmother died but the fact she kept feeling like she was falling could possibly mean her blood pressure was dropping to dangerously low levels. Which means her heart may have been slowing down to a stop. The fact that we pass out when our blood pressure goes very low likely also means your grandmother died a very peaceful death. My best to you on your loss.
Firstly, I am so very sorry for your loss and it is a terrible event indeed. Look, mate, I have no clue about what happened to your grandmother and I very highly doubt that anyone here does. You might be better going to the Undiagnosed Symptoms Community, have a look for it and you might have a better whack. I am so undeniably sorry for your loss, but the Communities on Medhelp are designed to look at that one topic and that topic only. Hang in there.