If it's a casino town, I'd expect there to be a bowling alley somewhere. So maybe there was a tournament on, and the favourite was about to win the game and get his cheque and trophy when the zombies ruined everything. So he's taken the trophy behind the bowling alley - to the place where the pins go.
You walk into the bowling alley and a bowling ball rolls towards you from the pins. So you walk up to the pins and after one of those little tense moments of silence, the apparatus that collects the pins comes down, picks you up and takes you to the bowler.
He's all mad at the zombies for ruining his crowning achievment in bowling, and he's mad that he'll never get to 'win' the title that was up for grabs because the event will never be held again after this. He's confused because he's sure that he would have won the trophy, but in reality he didn't because the match wasn't concluded.
So he picks you to play against, but he's gone a bit crazy sitting here with the trophy, so once you play a little bowling minigame and beat his over-confident ass, he pulls his piece out on the lanes, and tells you "you're entering a world of pain".
Commence boss battle!