I actually don't know how you got it to open an existing drawing at all.
Typically, Autocad creates a new drawing during startup, unless you have STARTUP=1. The new drawing is created from your QuickNew template, as specified in options. If you have no QNEW template specified, Autocad creates a new "empty" drawing, using no template.
I didn't even know it was possible to get Autocad to open a specific drawing at startup.