I believe we can rule out old age as the cause of death. Viera are confirmed to live three times as long as Hyur, so let’s say 300 tops. Erenville is confirmed to be 25 years old as of Dawntrail, thanks to the Eorzea Encyclopedia, and Cahciua was one of the warriors that sealed Valigarmanda 80 years ago. Let’s say hypothetically she herself was 25 when they sealed Valigarmanda: she would then be 80 when Erenville was born, 105 when he returned to Tural, and then 135 after the 30 year time skip caused by Alexandria. So barely middle aged for a Viera.

My theory is that given her “adventurous” nature, she may have been out in the wild and ran out of Soul Cells, perishing by creature attack. Upon becoming an Endless and learning about Living Memory, she found a way to control one of the robots in the real world so she could gather a following and try to stop the inhumane practices keeping the Endless “alive”.