97% to 99% of the chlorine in the pool is bound to Cyanuric Acid (CYA) and essentially inactive. It does not kill algae or pathogens. It's basically chlorine held in reserve and released as the active chlorine that is unbound to CYA gets used up. So the active chlorine level is proportional to the FC/CYA ratio. Trichlor pucks are a chemical combination of chlorine and CYA where for every 10 ppm FC added by Trichlor it also increases CYA by 6 ppm. The FC gets used up but the CYA remains behind so it builds up. You were maintaining your FC level, but the CYA level kept climbing so the FC/CYA ratio drops which means the active chlorine level that prevents algae growth dropped.
So long as the FC/CYA level was high enough to kill algae faster than it could grow, you didn't notice any problems, but once that ratio got too low (because the CYA got too high at a fixed FC level) then the increased algae growth started to consume chlorine faster. If the algae growth rate is slow, then people notice this as a mysterious increase in chlorine demand and they add more tablets faster to try and keep up. In your case, your algae growth rate may be faster so instead the FC level started to drop, the algae grew even faster, and then it all falls apart quickly as the algae use up all the chlorine in the pool and grow to their heart's content. It takes algae about 3 to 8 hours to double in population under ideal conditions so it really took several days for this to occur, but algae aren't always visible until there is enough of them to clump into visible pieces. Even using the conservative 8 hour doubling time, that means over 24 hours the algae population could have doubled three times so been 8 times as much algae so certainly the day before you might not have noticed much chlorine loss nor visible algae and then the next day seemingly out of nowhere you've got no chorine and have visible algae.
The Taylor book and the pool industry in general do not properly understand the chlorine/CYA relationship. So whenever you see a quote for an FC range independent of CYA level, it means absolutely nothing (i.e. it just demonstrates ignorance). FC is a measure of the chlorine reserve or capacity, not it's active strength in the pool. CYA doesn't just protect chlorine from degradation from sunlight, but also significantly moderates chlorine's strength. What is shown in the Pool School
Chlorine / CYA Chart are constant levels of active chlorine (hypochlorous acid) for various CYA levels and types of usage (i.e. minimum FC to prevent algae growth vs. killing algae quickly in a SLAM). Since algae are much harder to kill than most pathogens, having enough chlorine to prevent algae growth also disinfects quickly enough to make the pools safe from bacteria, viruses, and other pathogens.
The reason for the high minimum chlorine level of 9 ppm is because the CYA level is 110 ppm. The minimum FC/CYA ratio is roughly 7.5% and 7.5% of 110 ppm CYA is 8.25 so rounded up is 9 ppm FC. However, the active chlorine level is the same as in a pool with 3 ppm FC and 40 ppm CYA. The rate of killing algae and pathogens and the rate of oxidizing swimsuits, skin, and hair is the same in both situations. Again, FC means absolutely nothing as a number on its own except to tell you if you have enough chlorine to not run out (i.e. you have enough in reserve). It does not tell you the strength of the chlorine in the pool. Technically, the active chlorine level at the minimum FC level (FC/CYA ratio of 7.5%) is the same as only 0.06 ppm FC with no CYA. Fortunately it takes a very low level of active chlorine to kill pathogens and prevent algae.
So you can imagine what it must be like to swim in a pool with even 1 ppm FC but no CYA in it. Many commercial/public pools, especially indoors, are like this and are one reason they are more unpleasant to use. Swimsuits, skin, and hair get oxidized well over 10 times faster in such pools. CYA is a good thing to have, but like much in life it must be in moderation.