Option A: the tavern is not really haunted. All the weird stuff going on are just illusions produced by a mage paid to scare the potential customers. The mage comes into the tavern disguised as a different patron every day. Who is paying him to do it? Mystary!
Option B: the tavern is built on top of an ancient cemetery, and spirits of the dead are outraged by this sacrilege. They can be placated for a while, but the only solution to get rid of them is to demolish the tavern. Of course, to learn this, the PCs must find a way to somehow communicate with the spirits (who are not necessarily human).
Option C: grandfather of the tavern's current owner won the tavern in a game of dice. After losing his tavern, its previous owner got endebted and finally killed himself, only to learn in the afterlife that he got cheated in the dice game. He now manifests in front of honest gablers to warn them or waits until a cheater retreats to his rooms and scares him to death.
As for the exact manifestation of haunting... let's see. exploding bottles, darts that fly through the air on their own, blood appearing in guests' glasses instead of their drinks, flying kitchen cleaver of doom, unnerving sound of rattling dice that wakes guests at night, maggots in food, rugs that wrap around people and constrict, grandfather clock with a really sharp pendulum, talking trophy heads above the bar...