There seem to be two meanings of “unexpected” not being differentiated here:

1) name-derived terms like Debian, or the French ‘poubelle’ in the comments, which have become genericized to the point where most of its users don’t know the derivation

2) a more interesting subset of (1), like PageRank, or Lake Mountain in the comments, where part or all of the name itself looks like a normal word appropriate for the situation. (a related concept is nominative determinism https://en.m.wikipedia.org/wiki/Nominative_determinism)