Well, I'm pretty sure I picked up the usage from a pre-med friend in college. There was one group of friends that coalesced over time who liked to form fake plurals according to rules of other languages. Hence "touri" (a *very* common usage in Williamsburg for obvious reasons), "sofae", "sodae", "Kleenices" with all E's pronounced long (we even picked up "Unices" from a prof), and adding "-en" to any single syllable word ending in X.

Of course, I had met most of that particular gang of friends in my Intro Programming class... but we didn't know hackerese at the time.

Apparently "boxen" is used by some subsets of the Pennsylvania Deitsch as well... not surprising as it's a German thing to do, but what's odd is that I didn't pick it up in high school! (As I did with "X" which is the plural of "egg.")