The yeast has become sentient and is fighting for labor rights

The King has become increasingly teetotaling and has levied an ever increasing tax on beer and spirits to discourage “sin” among the people. In reality he has a god trapped under the castle and the only way to keep it contained is to keep it perpetually drunk. However its tolerance is increasing and now its dependence has reached a point where, if cutoff, its “DTs” could destroy the kingdom. The wine cellars have been totally emptied. Workers in the palace have become suspicious that the vast grates into which they empty confiscated barrels don’t, in fact, lead to the river.

Beer is essential 4 things; grain, hops, water and yeast.

Grain: cartels limiting shipments out of the grain basket.

Hops: A group that is very “anti drug” destroying hops since they come from the same category of plant.

Water: an addictive adulterated is being added, so they can charge more.

Yeast: someone figured out how to combine “speak with animals” with “speak with plants” letting them speak with fungi, convinced the yeast to stop working.

Depending on how medieval the setting is, hops were not the common bittering in many beers, it was a mix of herbs called gruit. If this mix’s ingredients are secret (or regulated), it would be easy to monopolize and drive the price up.

The price may be going up due to the cost of some profane adulterants being added in ever greater quantities…

A strange new product has come onto the market at the same time that someone has bought up the stocks of beer and grain.

The new brew has wonderful effect but its flavor is odd and ends with a very deep sleep. It’s cheap and widely available.

Turns out that some mind flayers or other psionic beast have been producing and distributing a concoction made from their own milk/extract.

The effects and deep sleep makes people easy and reusable prey.

Bit mundane underlying logic which can of course be embellished by the characters you populate it with: a brewer with connections to the royal court has acquired a monopoly by lobbying for brewing to only be allowed under royal warrant. They have managed to gain regulatory capture through their connections and all the other brewers are getting raided and their inventory flushed. Now the ‘royal’ brewer can set much higher prices without any danger of being undercut.

Or you could work it as some variant of beer purity law

All the process that make beer happen originate from the God of Spirits. His partner recently broke up with him as they are going to the Far Realms and don’t feel it was fair to keep it long distance. If anyone in the land wants beer again, they’ll have to act like some friends and make him smile again.

a malicious magical being is siphoning magical energies from the brewing of potions, but does not distinguish between witches and alchemists making real potions, and the brewing of beer; humanoid creatures are just as strange and eldritch to this being as it is to humanoids, and the processes seem similar enough. alternatively, the brewing of beer is actually very slightly magical, and beer is brewed in such higher quantities than potions that even the very small proportion of magic it can take from that is still more net energy than it can get from actual potions. this disturbance to the brewing process is making many batches of beer taste foul, and the brewers cannot sell them; supply is low and ingredients are being wasted, driving prices up. potions are also being affected, becoming inert or harmful instead of the intended effect, but this has not been noticed by the general public because they hardly ever interact with magical potions. also because people who make potions are trying to keep a lid on the situation to avoid suspicion from the public who are already wary and distrustful of them. (not familiar with the setting you mentioned, so maybe some of these things don’t quite fit, but could be a jumping-off point)

Lab engineered barrel blight is cutting off supply for brewers.

The king is charging a bubbles surtax.

Yeast is secretly being harvested from goblin tears, and the supply is running low after a goblin jailbreak.

A blight brought on by mystical means beyond the control of the hapless out of town merchant who invoked them solely for the opportunity to corner the market with his trove of inferior beer he couldn’t sell elsewhere. Once the players figure out the merchant is responsible, it becomes apparent there’s a far greater threat in the supernatural causes of the blight and they now must face off with them.