When and why did people begin using crosses as grave markers?

Answer 1

This is the answer, though many will not like it. The earliest crosses were placed on graves in the 12th century to keep the dead in their graves. People in those years were terrified of the Dead.

There were also people buried alive that broke out of their grave. They were, for the most part, insane from the ordeal. The clergy said they were possessed by the devil.

Meanings and usages do change in a thousand years.