I'd speculate that Los Alamos purchased the Unigraphics software and one of the Draftspersons who was learning the soft ware at the time decided to do some extra training time and draw out the Enterprise as its subject matter. Some of us Drafting types are huge Star Trek fans and we enjoyed Franz josephs blueprints so much that we became Draftspersons because of them.
The Unigraphics person training him might have liked the subject matter so much that they asked for a copy of the file which was used by the Unigraphics company then distributed as a sample file with each copy of Unigraphics sold.
All speculation but I bet you that this is the most logical answer that you will get. Short of tracking down the draftsperson who actually drew the computer file.