Easy, your alotted memory from your server was completely used... You had used 8388608 bytes (8 megabytes) and the script tried to use 3383773 additional bytes (3.2270174 megabytes).
Your webhost would have to increase the php memory limit which would be doubtful in most cases (16mb is a nice round upgrade number to request).
Your best bet however wouild be to run backups from command line, they're much faster and don't rely on the php variables.
