What happens when you die of an aortic aneurysm?

My grandma passed away on Monday. They found her body lying in the kitchen yesterday afternoon and they think she collapsed when she was getting her morning coffee. At Christmas time in 2014 she was diagnosed with her aneurysm and the doctor told her that with surgery she had a 50% chance of surviving, and without it, she had about two years left. She didn't get the surgery and two years and about three months later, she's gone. My mom says she must have been dead before the hit the ground. Is this true? What is going on inside the body when an aneurysm happens, and what causes it to finally burst? Did she suffer any pain? How and why does this happen?