If CYA levels are rising in a swimming pool, it's because CYA is being added to the pool. It does not increase naturally.
Cyanuric acid (also called stabilizer or conditioner) can be added to water manually or through stabilized chlorine products.
Trichlor and Dichlor (stabilized chlorine)
The most common reason for CYA levels rising is the use of stabilized chlorine. Specifically, Trichlor (trichloro-s-triazinetrione). Also known as trichloroisocyanuric acid, this chlorine type is extremely popular in the pool industry. It is most commonly in the form of 3-inch pressed tablets. 50-55% of these tabs (by weight) are cyanuric acid.
One pound of trichlor in 10,000 gallons of water increases the CYA level by 6-6.5 ppm.
(In metric, 0.45 kg of trichlor in 38,000 liters of water increases the CYA level by 6-6.5 mg/L).
A similar product is called Dichlor (Sodium dichloro-s-triazinetrione). This is not made in pressed tablet form like trichlor, but it has basically the same amount of CYA by weight (50-55%). Dichlor is much less common in swimming pools than trichlor, but its impact on CYA levels is the virtually identical. The difference is dichlor is primarily only used on an as-needed basis for shocking the pool, as opposed to a primary chlorine type.
Cyanuric acid (granular and liquid)
Commercial pools on non-stabilized chlorines like liquid chlorine and cal hypo often manually add CYA. This is usually done using a dry granular CYA. There are also liquid CYA products available, though they are more expensive and usually used on residential pools only. For larger pools, granular CYA is the product of choice.
If your CYA levels are rising, there is a source of CYA being added to the pool. If you did not add granular or liquid CYA to the water, check what type of chlorine the pool is using. If its trichlor or dichlor, there you go.