I'm sure someone will do the maths...but not me.
Bother! Now you've made me think about it. And, embarrassingly, a simple answer doesn't seem that hard.
Assuming voting is distributed uniformly across the existing survivors, I guess the chances would be 1/(13*12) = 1/156, or about 0.6%. Uniform distribution is probably a fairly poor assumption, though, since some contestants seem to attract more attention than others at any given time. Not sure what effect that has in this case.
EDIT: Although I didn't account for the fact that it was also a cross-post, which is what made it stand out. Those are uncommon, but not rare, let's estimate it is 1/20 (i.e., a crit fail
). That would bring it down to 1/3120 or about 0.03%. So, yeah, I guess that was somewhat remarkable, but not entirely unexpected over all Survivor threads.
EDIT2: And that all assumes that choice of up-vote, choice of down-vote, and cross-posting are all independent. Probably fair for cross-posting vs the choices; up-vote vs. down-vote probably not so much.