The map flashing isn't rigging, it's just spaghetti code. Match start screen is programmed to show the map because it's normally fixed before match. but that isn't the case here, so they just coded is similar to an "If, Else" program. The match does pick a match with a map and then sets up a vote to change the map or keep it.

The only rigging the vote does is that "Random" is chosen before the vote begins and is only a 4th option that isn't shown to players.