Is the boss spawn completely random?

In Killing Floor 2, the boss of a game can be either Hans Volter or the Patriarch. The Patriarch is objectively more difficult than Hans Volter, so I was wondering if anything affects which one will spawn.

That is, is there a 50/50 chance for either of them to spawn, or does the difficulty level make the Patriarch more likely to spawn, for example? I have noticed that I occasionally fight the same boss two games in a row, so it's not a simple alternation.