rpg

Someone is stealing the grain to feed an invading army.

A brewer cabal has employed ruffians who rob taverns and smash up the kegs, driving the prices up.

The crown is trying to keep it a secret, but there's a flesh-eating disease that is spreading throughout the kingdom, and it incubates in the alehouses. Taverns and breweries are being shuttered all over with only an offhand explanation. When ingesting the infected brew, the host will be consumed painfully and noisily from the inside.

A "hero" fresh from the dungeon throws his weight around and threatens that bad things might happen if he doesn't get paid protection money. He found a magic item that makes the danger clear.

Ale Elementals (Alementals)!

There is a Drunken Master Monk so powerful that they are weakening the barriers between the material plain and an outer plain of booze.

This manifests as inebriated Alementals appearing in brewery's and distillers and causing havoc before dispersing back to their home plane.

....

Less sinister, but potentially hilarious. (Cause could be changed if you wanted something more sinister)

Here's a boring mundane reason that could be fun. Ancient Egyptian pyramid builders were given beer as payment.

You could have the local evil person have an army of builders building some evil building, supplementing their pay with beer. Such a massive work force would require a lot of beer siphoning off most of the local supply. As the beer supply dwindles, the price increases.

Government official is getting extorted by [a path to the actual plot], but they own a vineyard and have the power to levy taxes. They tax beer to drive demand for wine.

Purely selfish self-preservation.

Dwindling supply due to bbeg creating a portal to some other plane that allows demons to come through and they get out of his control and just decide to get hammered 24/7

Affordability is essential to societal stability. Some cabal against the city government is pressuring the brewers, or the taverns, or the brewers guild, to increase beer prices. This can be made through taxes, fees, extortion... The extra money is siphoned to other actions to destabilise the city or to build a army. This can be done by kidnapping relatives to innocent people, corrupting government/guild officials, mind control...

The real motivation can be a noble dispute, a revolutionary uprising, a foreign plot to facilitate a conquest...

Make the beer magically cursed so people want to pay whatever price to get more.

Slurm Queen upped her fee

Constructed scarcity? The brewers pour out the beer per night on behalf of insert local rich person to make the prices higher, and when that has happened they then secretly will produce more to earn A GAZILLION gold.

(Based on a conspiracy theory i heard from a wacky guy at a party)

The town's blacksmith has been coerced into cooperating with a whimsically evil group of enchanters -- any weapon or accessory they work on secretly gets a nefarious spell put on it that makes tavern keepers overcharge you. It's part of a conspiracy to crash the ale economy so the enchanters can take over the taverns and jack up the prices for real! Or... maybe that's just what they want you to think and they're actually going to enchant the ale too with a more powerful magic that only works via ingestion -- allowing the enchanters to mind control the intoxicated adventurers and get them into incredibly silly misadventures that they can watch remotely as entertainment!

A powerful fae has charmed the largest distributor in the city, such that every barrel of ale he sells, he charges one more piece of silver than the last barrel. So prices are steadily climbing, and everyone thinks it's just the distributor being greedy. The fae's true agenda could be whatever makes sense for your larger conspiracy.