I mentioned this case under the category of "branching out", as his was the second cadet branch to break off from the family of King Christian IX of Denmark. But this is an umbrella term, and you're quite right that branching out can happen in a variety of ways. Aside from election, it can happen through bribery, conquest, etc.

If I understand correctly, the Norwegian parliament first considered another candidate -- Prince Carl of Sweden, second son of King Oscar II, the very sovereign under whose reign Norway separated from Sweden and acquired full autonomy. Either parliament rejected the move, after deliberation, or the prince (perhaps out of loyalty to his father) declined the offer (assuming it was made).

I'm not sure about the full details. But because it was not a case of succession by hereditary right (like in 1892 Great Britain, where the second son of the heir apparent replaced a deceased older brother as second and direct in the line to the throne) in 1905, this was clearly a case of an unexpected accession of a younger son.