I'd say "unprotected" is a slight exaggeration. Underprotected I'd agree with, as a single warrior doesn't give much protection against an archer. And there is the slight detail that it was unprotected the turn before it was attacked -- I had the settler on go-to and had to shut it off to keep it from walking straight to that archer, and the warrior caught up just in time to be slaughtered.
At that early stage, an unprovoked attack didn't
seem that likely, at least to someone playing his
very first MP game.