I believe this is caused by the map vote being called with a couple of minutes remaining. However, on maps where the map changes after a certain number of rounds i.e. Goldrush, then there is still time on the clock when the map changes, and hence sometimes a vote never gets called.
If this happens, then the next map in the rotation will be loaded. You should only see happycow after a certain other map - not always when there is not a map vote.
It's a rare occurrence that the map vote never happens, but maybe we should move it to be earlier.
We could also install a rotation handler plugin, which would enable us to vary the rotation based on player numbers/time of day etc. With this, the end of map vote could possibly be removed, and with rtv as an option for voting in more obscure maps.
Just thoughts.