This is one possible way it could have happened.
I do not own the xenomorphs, and they belong to all their proper copyright holders. I'm just borrowing them for a fanfiction, as it were.
Artwork: mine
Original story: http://ravencorinncarluk.blogspot.com/2012/04/fridayflash-xenomorph-infestation.html
Music: "Malicious" Kevin MacLeod (incompetech.com)
Licensed under Creative Commons: By Attribution 3.0 License
http://creativecommons.org/licenses/by/3.0/